MEMOIRS
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BRISBANE VOLUME 36
10 AUGUST, 1994 PART 2
SCALLOP FISHERIES IN SOUTHERN AUSTRALIA:
MANAGING FOR STOCK RECOVERY
WILL ZACHARIN
Zachann, W. 1994 08 10: Scallop fisheries in southern Australia: managing for stock
recovery, Memoirs of the Queensland Museum 36(2). 241-246. Brisbane. ISSN 0079-
8835,
Scallop fisheries in southern Australia are showing signs of stock recovery after a
period of
low abundance. The recovery has been sporadic and slow although large areas of the fishing
grounds have been subject to little or no fishing for upto 5 years. New management strategies
designed to encourage stock recovery and promote sustainable harvests in the future are in
place. Management strategies and fishery monitoring programs are presented.
Will Zacharin, Sea Fisheries Division, Department of Primary Industry and Fisheries, GPO
Box 619F. Hobart, Tasmania 7001, Australian, 15 April, 1994
There are 5 distinct commercial scallop fishing
zones m southern Australia : Port Phillip Bay and
Lakes Entrance in Victoria; the greater area of
Bass Strait (known as the Central Zone), the 20
nautical mile zone around the north coast of Tas-
mania called the Tasmanian Zone, and the east
coast of Tasmania (Fig.1). They are geographi-
cally distinct in terms of their historical catch and
fleet dynamics. Management is under the control
of 3 separate authorities; the Victorian and Tas-
manian State Governments and the Common-
wealth Government (Australian Fish Man-
agement Authority). Three different management
strategies are operating.
The Bass Strait Scallop Consultative Commit-
tee (BSSCC) formed in 1991 to develop a rational
management plan for scallop fisheries across
Bass Strait, This was the second time in the
fisheries’ history that such a process had been
attempted (Zacharin, 1990, 1991), An earlier plan
developed by the Bass Strait Task Force which
recommended that the fisheries" jurisdiction be
split between Victoria and Tasmania was not
effectively implemented (Zacharin, 1990).
Fishermen and managers recognised that future
harvesting strategies needed to be based on cur-
reni biological knowledge of the species (in
regard to reproductive maturity and growth
rates), fleet dynamics and the need for economic
efficiency. The committee drafted a management
plan with 5 main objectives: I, to control fishing
effort to a level which is consistent with the
current state of knowledge of scallop stocks; 2, to
encourage investigation and modification of the
most appropriate fishing equipment and fishing
practices to improve catch efficiency and to min-
imise damage to the scallop beds; 3, to allow
further scientific and other data to be collected so
that management decisions can be based on a
sound understanding of biological and operation-
al characteristics of the fishery; 4, to allow an
effective level of recruitment to the fishery by
prohibiting the taking of scallops of «80mm with
a view to allowing adult stocks to complete at
least two major spawnings before harvest; and 5,
to allow participants to maximise their return
from harvesting the scallop resource. (Bass Strait
Scallop Management Plan 1952, Commonwealth
Fisheries Act 1991).
The resultant management strategy combines
input and output controls to restrict the number of
fishers; to prohibit the taking of small scallops; to
contro] scallop landings and to provide a level of
profitability to the fleet.
CONTROLS ON FISHING
In the past, both the States and the Common-
wealth restricted fishing activitics by imposing
input controls, such as closed seasons, size limits
and dredge restrictions. Over the past 4 years
there has been a shift towards output controls as
they are perceived to be more effective in manag-
ing catch and controlling quality, provided that
the necessary level of monitonng and enforce-
ment is present. A size limit of 80mm at widest
diameter, however, still exists. The two main
strategies of the new management plan for the
Central Zone of Bass Strait are the * 20% trashing
rate’ requirement and the 'two-spawnings'
criterion. The 20 % trashing rate was designed as
a yield optimisation strategy, through limiting the
capture of, and minimising incidental mortality to
small (<80mm at widest diameter) scallops. The
‘two spawning’ criterion is a parallel manage-
ment requirement designed to allow scallops two
242
MEMOIRS OF THE QUEENSLAND MUSEUM
FIG.1. Southern Australian scallop fishery divided into 5 distinct zones. Port Phillip Bay (1), Lakes Entrance
(2), Bass Strait (3), northern Tasmania (4), and eastern Tasmania (5).
major spawnings prior to their being fished,
without regard to size. Thus scallops need both
to have spawned twice and have less than 20%
of the catch smaller than 80mm at widest
diameter prior to their being fished.
The trashing rate is the proportion of small
scallops discarded over a fishing ground during
commercial operations. If more than 20% of the
catch landed on the sorting tray is being retumed
to the water, fishermen are required to cease
fishing in the area until scallop size increases.
This is not difficult for the majority of Bass Strait
scallop beds as they are usually of the one size or
age class. However, in the event of two age
SCALLOP STOCK RECOVERY, SOUTHERN AUSTRALIA
classes being mixed in the one area, a 20% trash-
ing rate is considered acceptable, having regard
for increasing mortality in the older age class, and
the potential of predators to significantly reduce
the remaining scallops on a fished bed,
Failure of fishermen to leave the area can result
in a 3 month closure to the whole fishery, This
closure can be implemented by the management
committee under a specific provision in the Bass
Strait scallop management plan.
Application of trashing rates are not new in
shellfish management. A trashing rate of not
more than 30% of landed catch was introduced
into the eastem U.S offshore clam fishery in
1983. The reason was lo prevent wastage due 10
excessive discarding and to mect minimum size
requirements (Murawski & Serchuk, 1989).
Two major spawnings from adults, prior to their
being fished, are considered essential if sufficient
reproductive output from the fishery 1s to occur.
Commercial scallops in Bass Strait have their first
major spawning in their second year {1+ age
class). However, fecundity is relatively low at this
age(R. McLouglin pers. comm.) and therefore
delaying the fishing until the scallops’ second
major spawning is desirable to increase the prob-
ability of some recruitment from that particular
age class of adult spawners. Restricting fishing
operations even further until a third major spawn-
ing has occurred cannot be defended, as natural
mortality is thought to be high in Bass Strait
populations after scallops reach an age of four
years, High levels of predation by starfish on
commercial scallop beds have been observed on
a number of occasions,
Delaying the time of first capture till after the
second spawning has a number of other benefits,
Scallops have another year's growth, which
results in the majority of the population reaching
a shell height 70mm (shell width 80mm), In-
dividual yields increase c.30%, and the landed
value of the fishery should nse, There is ап as-
sumption that there is no rapid increase in natural
mortality. A yield optimisation mode! needs to be
completed to support this assumption.
The crux of the management plan ts, if the
trashing rale is below 20%, then it can be assumed
that the bed should be fished until it is no longer
economically viable to continue. After fishing
scallops will stil] remain in the area but at a low
density.
CATCH RESTRICTIONS
The Buss Strait fishery opens on 1 April of each
243
year and closes in late December. This summer
closure protects juveniles from dredge damage
and stops scallops with poor meat condition being
landed, In most years, post spawning meat and
gonad condition does not improve unti] March,
Scallop landings are subject to ‘quota’, set per trip
or forinightly. At present the quota is 150 units
per fortnight; a unit being a black polypropylene
onion bag measuring 900mm x 580mm and
having a volume of 0.08 m?. This measure owes
its derivation to the past availability and suit-
ability of onion bags for landing scallops.
While the fortnightly quota does reduce fishing
effort to some extent, this is not its primary pur-
pose, It is à marketing too] which provides for the
landing of quality scallops and prevents wastage
due to time delays in Janding and processing
larger volumes. It prevents a ‘gold rush’ event, as
occurs when there isa competitive total allowable
catch, The catch quota was agreed through
negotiation between Government, fishermen and
the processing sector, If costs of fishing rise and
landed price falls or even remains steady, it 3s
possible for the industry to re-negotiate the catch
quota at any time. Profitability of the fleet is a
main objective of the management plan.
Catch is also controlled in the Victorian and
Tasmanian Zones. In Victoria a weekly catch
limit is currently operating, while in Tasmania, a
‘per tip’ limit will continue to operate when
fishing recornmences in the future.
QUOTA MONITORING AND CATCH DATA
Each unit or bag landed must have a plastic
colour-coded tag attached. Tags are issued each
month in advance by the Australian Fish Manage-
menl Authority, Unused tags are retumed as а
cost saving measure and are re-issued the follow-
ing year but in a different month. Numerical
coding also changes each month and year to
ensure unused tags will not be held over from year
10 year, The tag system allows efficient monitor-
ing and enforcement óf the quota, and in provid-
ing a validation system for scallop landings
through the processing sector.
А new logbook introduced in 1992 15 based on
a 7 x 7 nautical mile grid. Retums are filled out
for each trip and data entered on a central com-
puter database in Hobart. The system will give à
better assessment of fleet dynamics, exploitation
rates and total landed catch from the Central
Zone. 1n the past the fleet has provided catch
returns without meaningful spatial data to the
State authority in which the vessel was based,
244
Consequently, no comprehensive analysis of the
fishery has been possible. Victorian and Tas-
manian fishery managers continue to collate their
own catch returns from the inshore 20 nautical
mile zones.
LICENSING
All 3 jurisdictional zones are теў entry
fisheries and no new licences will be issued.
There are 165 vessels licensed to fish in the
Central Zone. Of these 73 are based in Tasmania
and 92 in Victoria. Licences in Victoria are trans-
ferable and have been for the better part of the
30-year history of the fishery. In Tasmania,
limited entry was not introduced until 1986 with
transferability following in 1992 (Zacharin,
1990), Central Zone licences are stil] non-trans-
ferable pending the development of options for
reducing the number of participants in the fishery.
Tt is desirable that the issue of transferability be
resolyed, as Central Zone licences cannot be split
from State scallop licences, which are trans-
ferable. It would be highly undesirable to create
a ‘third’ scallop fleet in the Central Zone of Bass
Strait. An important objective of the licensing
policy is to have all the Central Zone licences held
by the State scallop fleets, as the inshore scallop
fishing grounds have historically provided the
bulk of the scallop catch, with the Central Zone
providing good catches intermittently.
FISHING GEAR
The southern scallop fishery uses tooth-bar
steel box dredges 2-4.5m wide. Protruding teeth
on the bars range from 2.5-15cm, depending on
the type of bottom sediment and the individual
operator. These dredges can cause high levels of
incidental damage and alternative designs are still
being investigated. Evidence from dredge trials
shows that up to 50% of scallops in the dredge's
path may be damaged, depending on the type of
bottom, length of toothbar, density of scallops
and fishing practices, Dredge efficiency can be
low, having been experimentally measured at 10
% (McLouglin er af,, 1991), Gear technology im-
provements are important to this fishery as any
reduction in incidental mortality and increases in
efficiency will reduce costs and increase yields.
EFFECTIVENESS OF MANAGEMENT
STRATEGIES
The new plan for the Central Zone has yel lo be
MEMOIRS OF THE QUEENSLAND MUSEUM
tested under rigorous fishing operations duc to the
low level of commercial fishing operations.
Mechanisms such as at-sea monitoring and shore
based market measurers will provide for a quick
response to any problems that anse with regard
to scallop size. The Victorian fishery has been
operating under à tag system for two ycars and
the scallop industry seems pleased with the
progress of this system.
Any management plan for the southern scallop
fishery should be complementary between Vic-
torian and Tasmanian authorities. The new plan
for the Central Zone goes a long way towards
achieving this; however, further gains may be
difficult because of the differences in fleet
dynamics between the two States,
The Victorian scallop fishery has a single
licensed fleet that is heavily depreciated and lar-
gely reliant on annual scallop fishing seasons, In
Tasmania the multi-purpose fishing fleet has
evolved with the majority of scallop licences
being on vessels with rock lobster entitlements,
Other Tasmanian scallop vessels are licensed to
drop-line, trawl or take shark during a closed
scallop season. These differences in dynamics
between the Victorian and Tasmanian fleets have
resulted in each having different economic con-
straints. The zoning of the Bass Strait scallop
fishery needs to be retained to enable the subtle
differences in management priorities to operate,
as appropriate for each State's fishing industry.
RESEARCH AND DEVELOPMENT
REQUIREMENTS
Six future research needs, identified for the
southern scallop fishery by Ihe Bass Strait
Management Committee, are listed in order of
prioritv: 1, confirmation that Bass Strait scallops
consist of a single stock; 2, development of an
efficient and reliable recruitment monitoring
technique to provide an index of annual spatfall;
3, development of statistically reliable survey
techniques for assessing biomass on individual
beds; 4, assessment of the overall impact of
predation by starfish (Coscinasterias sp.) on scal-
lop populations; 5, investigation of recruitment
enhancemenvsea ranching of scallops as per the
New Zealand model; and 6, investigation of dif-
ferences in growth rates and fecundity schedules
for scallops in different regions of Bass Strait
(Bass Strait Scallop Management Committee
1992, mimeo),
The second priority is important in providing a
measure of success of the management plan,
SCALLOP STOCK RECOVERY, SOUTHERN AUSTRALIA
specifically the iwo-spawning criterion. A
recruitment index also provides early warning of
recruitment failure or 'above-average' recruil-
ment success.
The impact of predatory starfish was
demonstrated to be of considerable importance in
1992. An identified scallop bed east of Deal Is-
land in Bass Strait was decimated by starfish
during a delay to fishing, in an attempt to conform
to the two-spawning criterion and improve scal-
lop yields. Further investigation of these
predators is necessary to prevent such an occur-
rence happening again.
PROGNOSIS FOR 1993 AND BEYOND
There has been a significant recovery of scallop
stock(s) in both Port Phillip Bay and off Lakes
Entrance in Victoria. A large settlement occurred
inthe spring of 1990 with subsequent recruitment
to the fisheries in 1992. Further settlement has
been observed in cach of the following years and
the fisheries are showing good prospects for the
next one to two years (Zacharin - pers. obs.), It is
ironic that the beds off Lakes Entrance (which
have been sporadically fished) have recovered
before the scallop grounds in Tasmania (where
the fishery has been closed for five years), In this
instance, total closure of the frshery has not lead
to any earlier stock recovery than has been ob-
served in Victorian waters, where fishing con-
tinued. However, there is no certainty that the
factors affecting recruitment off Lakes Entrance
apply over a much wider area, and no conclusions
can be made in terms of management for stock
recovery.
Recent exploratery excursions into the Central
Zone and the northern Tasmanian Zone have
shown that juvenile scallops are present over a
wide area. If these juveniles successfully recruit
into the fishery in 1993 and 1994, an economical-
ly viable fishery will again operate in the Tas-
maman and Central Zones,
LESSONS TO BE LEARNT
The recovery of the Victorian scallop grounds,
through what appears in Port Phillip Bay to be due
to an enormous settlement event in 1991, is dif-
ficult to explain. The residual stock in the hay was
apparently at an all time low at 19 million, but one
of the largest recorded settlements has occurred.
The estimated abundance is in excess of 800
million scallops (D. Molloy, pers. comm.). This
is another example of the critical influence of
245
environmental variables on successful spawning
events, settlement and subsequent recruitment.
Stock/recruit relationships of P. fumatus in
southern Australia appear to be extremely noisy
if they exist at all. These observations support the
new strategy of allowing two major spawnings
before harvesting. particularly in the offshore
fisheries where retention of spat over scallop
grounds will be more variable than in the
enclosed environs of Port Phillip Bay.
It is important to remember that the scallop
fleets of Victoria and Tasmania are different in
terms of their level of capital investment, vessel
specifications, fishing patterns and reliance on
the scallop resource for income. No hard and fast
management plan across the three existing zones
will be successful in meeting both States’ ad-
ministrative and economic requirements. Com-
plementary management plans that take account
of these differences are preferable to continued
friction between the two State based fleets. One
needs to be aware that the majority of the histori-
cal catch has come from the state 20 nautical mile
zones, the Central Zone resource being one of
sporadic opportunity.
Change for its own sake can be a destructive
policy. The success or otherwise of the current
management plan operating in the southem scal-
lop fishery should be assessed before major chan-
ges are contemplated. Feedback on the effects of
the trashing rate and two spawning strategy will
not be evident for two to three years, With the new
logbook providing better spatial information on
catch, an integrated catch datahase system and
progression towards developing a recruitment
index or forecasting system, management of the
scallop resources in southern Australia can only
improve,
The Australian Fish Management Authority
will probably relinquish responsibility for the
Bass Strait scallop fishery 1n 1994 and leave joint
management to the Victorian and Tasmanian
agencies, A jurisdictional line would be drawn
for the purpose of monitoring and enforcement
responsibilities. The existence of remaining Bass
Strait permits for the Central Zone which are not
attached to state scallop licences may impede this
process,
ACKNOWLEDGEMENTS
Thanks are due to the many members of the
Bass Strait Scallop Consultative Committee for
plugging away at а new management plan after
ten years of mectings at all manner of manage-
246
ment committees and venues. I would also thank
two anonymous referees for suggesting improve-
ments to this manuscript.
LITERATURE CITED
COMMONWEALTH FISHERIES ACT 1991: BASS
STRAIT SCALLOP MANAGEMENT PLAN
1992.
MCLOUGLIN, R.J, YOUNG, P.C., MARTIN, R.B. &
PARSLOW, J. 1991. The Australian scallop
dredge: estimates of catching efficiency and as-
sociated indirect fishing mortality. Fishery Re-
search 11: 1-24,
MEMOIRS OF THE QUEENSLAND MUSEUM
MURAWSKI, S.A. & SERCHUK, F.M. 1989.
Mechanized shellfish harvesting and its manage-
ment: the offshore clam fishery of the eastern
United States. Pp. 479-506. In Caddy, J.F., (ed.),
‘Marine invertebrate fisheries’.
ZACHARIN, W.F 1990. Scallop fisheries manage-
ment: the Tasmanian experience. Pp. 1-11. In
Dredge, M.L.C., Zacharin, W.F. & Joll, L.M.,
(eds), ‘Proceedings of the Australasian Scallop
Workshop Hobart 1988'. (Tasmanian Govern-
ment Printer: Hobart).
ZACHARIN, W.F. 1991. Slow recovery for Bass Strait
scallops. Australian Fisheries 50(1): 28—30.
POPULATION AND BIOLOGY OF THE COMMERCIAL SCALLOP (PECTEN
FUMATUS) IN JERVIS BAY, NSW
HECTOR R. FUENTES
Fuentes, H.R, 1994 08 10; Population and biology of the commercial scallop (Pecten
Jumatus) in Jervis Bay, NSW. Memoirs of the Queensland Museum 362): 247-259.
Brisbane, ISSN 0079-8835,
Following a peak jn 1981/82, the commercial scallop fishery in Jervis Bay declined to the
point where the dredge fishery finished in 1984 and the dive fishery in 1989-90. Despite
past economic importance, little information was available on the biology of Pecten fumatus
in Jervis Bay, Two small, low density, scallop beds in the south and north of the bay had
different densities. Most scallops were found at depths of 15-20m. Density increased from
1990 to 1992. Recruitment events occurred in November 1989, November 1990 and in March
1991, Three groups that may be age classes 0+, 1+ and 2+ years were identified in each year.
Lower reproductive activity occurred from December to March and 3 or 4 periods of higher
activity occurred between April and December, suggesting multiple spawning behaviour.
There was poor correlation between water temperature and gonad index, but significant
correlation was found between increasing numbers of parasitised scallops and period of
increasing water temperature. Main settlement occurred from November to January. It
appears that there are factors which prevent successful settlement in locations other than the
two main beds. There was a greater settlement at depths of 8-14m.
Hector R. Fuentes, NSW Fisheries, Fisheries Research Instilute, Р.О. Box 21, Cronulla, New
South Wales 2230, Australia; Present address; Queensland Deparment of Primary In-
dustries, Northern Fisheries Centre, P.O. Box 5396, Cairns, Queensland 4870; 23 May,
1994.
In 1988 à study was initiated in Jervis Bay to
provide baseline information on commercial
molluscs for a management plan. Four species
are of economic interest but this study deals only
with the commercial scallop (Pecten fumatus),
which has been the basis of an intermittent fishery
since 1970,
Historical information (Hamer & Jacobs, 1987;
Young & Martin,1989) suggests that commercial
scallop harvests in NSW were high in the early
1970's, but there is no indication of the total
production in Jervis Bay. However, during the
fiscal year 1981—1982 the fishery reported 2,822
tonnes (Stewart et al.,1991) for NSW with 1,329
tonnes comming from Jervis Bay. Anecdotal in-
formation suggests that up to 35 dredge boats and
an unknown number of commercial scallop
divers were operating in the bay during the 1981—
1982 peak in the fishery. The dredge boats
stopped operating in 1983-84 when the scallop
fishery became uneconomic. The divers persisted
until the end of 1990, although they have har-
vested <10 tonnes per annum in recent years.
Recreational divers harvest scallops in Jervis
Bay, hit there are no catch estimates. Due to the
low density of scallops, the NSW Department of
Fisheries recommended a total closure of the
fishery from November, 1991 to June, 1994. to
allow stock recovery.
The aims of population level work were to
provide information on distribution, abundance,
size composition and settlement. The aim of work
at the individual level was to increase the
knowledge of the reproductive cycle.
MATERIALS AND METHODS
POPULATION SURVEYS
The distribution and abundance of scallops
(Fig.1) were estimated during grid and transect
dive surveys during 1989-1991 (Fuentes et al.,
1992). Random transect surveys in February
1990 and 1991 (Fuentes et al.,1992) examined
populations in areas identified during grid sur-
yeys as having high concentrations of scallops. In
April 1992, 35 transects were allocated to each
area. In the transect surveys, scallops were clas-
sified according to size: small (flat shell
«30mm), medium (30-60mm) and large
(760mm). The length categories were chosen on
basis of the length-age relationships (Hamer,
1987). Transects containing scallops were
grouped into: high density transects (70.1 seal
Im?) and low density transects («0.1 scal/m?),
Population size structure was based on length-
248
Jervis Bay
Honeymoon Вау r^
x
E
2
F
z
FIG.1. Location of Jervis Bay.
frequency surveys in the Murrays Beach bed
(Fuentes et al.,1992). Only data from surveys
between September, 1989 and August, 1991 are
included herein. All samples were taken from the
Murrays Beach bed by 2-3 divers who collected
all scallops that they saw during 40—50min dives
in 17-20m. The collections were assumed to es-
timate the actual length- frequency distribution of
scallops in the bed.
REPRODUCTIVE BIOLOGY
Regular collections of 50 scallops of commer-
cial size (>65mm) were taken by SCUBA divers
from the Murrays Beach bed. Monthly or
fortnightly samples were taken according to the
state of the gonads. Based on the assumption that
the gonad weight of mature individuals changes
in relation to total body weight during the breed-
ing season, a gonosomatic index (GSI) was used
as indicator of reproductive condition (Grant &
Tyler,1983; Barber & Blake,1991). A GST was
calculated for each nonparasitised scallop: GSI =
(GW/BW — GW) * 100, where GW is the gonad
weight and BW is the body weight in grams.
Mean GSI values and frequencies of parasitised
scallops were correlated to weekly bottom
temperatures from near the collecting site.
MEMOIRS OF THE QUEENSLAND MUSEUM
Macroscopic and microscopic examinations of
gonads were conducted to investigate scallop
reproductive behaviour and to implement an easy
and rapid technique to assess the reproductive
condition of scallops. In the macroscopic study,
dissected gonads were classified into 7 stages:
Immature, Developing 1, Developing 2, Ripe,
Spawning 1, Spawning 2 and Parasitised. In the
microscopic study, the female sections of the
gonads were classified into 9 stages: Immature,
Early development, Ripe, Partial spawning, Ex-
tensive spawning, Resorption, Resting and
Parasitised,
SETTLEMENT
A longline system with collector bags acting as
artificial substrata was used to study the spatial
and temporal characteristics of scallop settle-
ment. From August 1989 to February 1990 settle-
ment was studied near the Murrays Beach and
Honeymoon Bay scallop beds. From August
1990 to February 1991, Plantation Point, Huskis-
son and Green Point were added for settlement
studies. For detailed descriptions of the sampling
design and location of collectors see Fuentes et
al. (1992),
“wo ON
P И ѓ
| i /
M d r
| »/ H )
(il i {
|^ | J
ү Н Y
4] |
MES 7
| 4
I H
| 1
[| Ае E
ERN T,
/ į ~
] f
y Н
j t /
MNA „
\ 4 Ty
\ É Lu
N [| j
MEA ;
\
=wGorman and ]оһп=шт 1972 N
~ Dames and Moore 1985. 0 5
© fuentes et al 1997 ы = d
FIG.2. Distribution changes of the commercial scal-
lop, P. fumatus, inJervis Bay. The straight line is the
boundary between State and Commonwealth waters.
POPULATION BIOLOGY, Р. FUMATUS, JERVIS BAY
249
(Butcher et al.,1981) and a
E ELM number of other studies
"A (Jacobs,1983; Hamer,1987;
O^ sos Hamer & Jacobs,1987; Wil-
|
| Bowen llam
/
и liams & Diver,1988; Fuentes et
al.,1992) indicated that P.
fumatus occurred throughout
the bay (Fuentes et al.,1990).
Commercial scallops were
primarily confined to Murrays
Beach and Honeymoon Bay
(Fig.2), with only few in-
dividuals observed elsewhere.
More scallops were found be-
tween 15-20m than between
5-15m. What constrained the
commercial scallop to this dis-
tribution is unknown, but
natural environmental chan-
FIG.3. Location of transects at Murrays Beach Bed in April 1992. Figures ges, fishing methods, over-
on the circles are transect numbers.
RESULTS AND DISCUSSION
POPULATION SURVEYS
DISTRIBUTION: The first assessments of commer-
cial scallop distribution including abundance
were obtained from the dredge surveys conducted
by FRV Kapala during the 1970 and 1971 peak
in the scallop fishery (Gorman & Johnson, 1972).
No further scallop investigations were conducted
in Jervis Bay until the fishery boomed again in
the early 1980's. At this time a dive survey
fishing or a combination of fac-
tors should be examined.
Dredging was the most common method of
harvesting scallops in Jervis Bay during the years
of intensive fishing. Although some studies have
alleged that dredging has no adverse effect on
scallops (Butcher et al.,1981), other authors have
suggested that dredges both cause considerable
damage to scallops that are left in the dredge track
(Caddy,1973, McLoughlin et al.,1991) or cause
detrimental changes to the bottom (McLoughlin
et al.,199], Riemann and Hoffmann, 1991) which
prevent or inhibit scallop settlement. However, it
TABLE 1. Sampling effort and abundance of scallops in the two main scallop beds in Jervis Bay during transect
dives in February 1990, February 1991 and April 1992,
|. MumaysBeach _ | — HoneymoonBay — | TOTAL —— |
| 1992 | 1990 | 1991 | 1992 | 1990 | 1991 | 1992 |
[memes | s | s [s px pss emm]
Ы мастава шэ | 20 | zwo | 2100 | zoso f 2,00 2.0 | амо | аю | s200
| 95 | 103 | 101 | ==
| m | m | m | m |
| 16 | as | m | 20 | з | 15 | 36 | 38 |
егш Poe. e a | же ш
pom T [юу |
| Transects with scallops | 19 |
| Total scallops
| Small scallops(<30mm) | 21 | 76 | 9 |
[Medium (31-6omm) |
| Large scallops (>60mm) |
Densi per m "
mm
аз mu sm M M Wm NE E. E
| s | a |29 | æ | m | в | no | 5 |
Мезон | та ] 75 | зз] эз | 27
[Mean scallops/allwansects| 43 | 43 | 390 | 12 | o9 | as | 24 | 26 |
ол» | 0.125 | oso |
| тош density (perm?) | 0071 | 0071 | osso | oom | oos | оовт | оо | ооз | Q075 |
2.6 21.7
| ооз! | бов | 0.124 | 0.096 | 0095 | 0577 |
include only transects with scallops.
183
[30 |
| 742 | 58 | s7 | 3466 |
250
FIG.4, Survey areas showing the differential abundance of commercial scallops
in the Murrays Beach Bed during the transect survey in April 1992.
is still not known if fishing technique was the only
factor responsible for a decrease in these
fisheries.
ABUNDANCE: Dredge surveys and dive surveys
provide similar estimates of scallop distributions
(McShane,1982; McShane & O'Connor,1982);
however, dive surveys yield more precise es-
timates of abundance although the estimates typi-
cally are lower, There were no dredge vessels
operating in Jervis Bay during the time of this
study, therefore, only estimates from dive sur-
veys were available.
The Murrays Beach bed typically contained a
relatively low density of scallops,
and the Honeymoon Bay bed was
even more sparcely populated.
However, a comparison of tran-
sect surveys in 1992 with pre-
vious surveys in 1990 and 1991
(Table 1) indicated differences in
the abundance of the three size
classes and an increase in the |
total number of scallops. At both
locations, the number of medium
size scallops were more abundant
in 1992 than in previous years.
which suggest improved recruit-
ment to the fishery,
In the 1992 transect survey at
the Murrays Beach bed, scallops
were found in only 23 transects.
| Итен Inland
[Densityitranseet| 0.130 | 0.223 | 0.037 | 0.039 | 20 | &o | 78 |
| high density |
Лом density | 0.035 | 0047 | 0.022 | 0006 | i2 | 60 |
MEMOIRS OF THE QUEENSLAND MUSEUM
The average number of
scallops in all transects was
0.99] scal/m? (SE =
+0.264). The location and
scallop density for each
transect (Fig. 3) identified
the E-W boundaries of the
bed, but it did not identify
the northern limit of the bed
which extends towards the
middle of the bay. Within
the bed, areas of high
(>0.1scal/ m?) and low
(«0.1scal/m?) density were
identified (Fig.4). The high
density area (2.3km?) con-
tained 4.20x10° scallops
while the low density area
(1.6km?) contained
c.0.06x10* scallops (Table
2). The bed occupied
3.7km?, and interpolation
among. the most external
transects containing scallops indicated that the
bed contained 3.66x 105 scallops.
This exercise was repeated at Honeymoon Bay
where 20 transects contained scallops (Fig.5).
The average number of scallops per m* in all
transects with scallops was 0.130 (SE = +0.039).
Plotting the location and density for each transect
defined the boundaries of this bed (Fig.6). The
high density area (70. 1scal/m?) was almost com-
pletely surrounded by a low density area
(«0.1scal/m?), The high density area (2.3km?)
contained 0,6] x 105 scallops (Table 2). The total
area of 6.0km? contained approximately 0.78 x
106 scallops.
Tesi redimi annes
Bl 220 » 10" оре
ES) бё e 10 Scallups
TABLE 2. Mean of scallop density and estimation of abundance at 2
locations in Jervis Bay during April 1992, Abundance estimates were
calculated using only those transects containing scallops.
Ау.
су ra a — — l
[Density/transect| 0.991 | 1.538 | 0.443 | 0264 | 23 | 37 | 366 |
| highdensiy | 1.865 | 2.633 | 1.097 | 0.349 |
| tow density | 0036 | 0049 | 0.023 | 0.006 | 11 | L6 | 0% |
95% confidence
limit
St | No. | Est. | Abun |
error of areg dance |
Upper | Lower Tran | (km )| (x10 )
sects
Murrays Beach
Honeymoon Ba
[0449 | 0097 | 0034 | 8 [23 | osi |
0.78
POPULATION BIOLOGY, P, FUMATUS, JERVIS BAY
Density
[ EP
È <0.1зср\/л©
О Noscallaps
j
af leny
l
!
/
17
23 m 2 +
Won О a o’ Honeymocn Bay
}
ГА
| ale
oS ) 3
EZ? [pira PU м
о air
‚лә Ө?
je.
0 5
La
km
—
FIG.5. Location of all transects at Honeymoon Bay.
Figures on the circles are the transect numbers in
April 1992,
SIZE STRUCTURE:The population size composi-
tion and progression of the modal size classes
(Fig.7) illustrate variations in size composition
through time. At least two size classes were evi-
dent in most samples. The number of scallops
increased in the last six samples and the highest
numbers of small scallops were also taken near
the end of the investigation.
The first recruitment observed during this study
occurred in November 1989 when two cohorts
were observed; one with a mode in the 28mm
class-size and a second in the 63mm class-size.
On the basis of Hamer's (1987) ageing criteria,
the first cohort could represent a 0+ age group
and the second a mixture of 1+ and 2+ age groups.
The two cohort structure seen in November 1989
and January 1990 persist until May 1990. From
July 1990 to September 1990, the population size
distribution was unimodal with no cohort com-
ponents.
A second recruitment pulse appeared in
November 1990. A few individuals of 28mm (0+
year of age class) suggest a small recruitment, or
less than that of the corresponding month in the
251
previous year (November 1989). The sample
from January 1991 showed the 0+ age group seen
in November 1990 at 43mm.
The sample from March 1991 showed 2 size
groups similar to those observed in November,
1989. This suggests that the main recruitment in
the second year of this study occurred 4 months
later than in the previous season. From March to
May 1991, the two cohorts were evident, but they
merged again by July 1991.
Small scallops are hard to see, and only one
sample (March, 1991) had any individuals
<10mm in shell length. Scallops «20mm shell
length were also found in few samples (Novem-
ber, 1989, January, 1990, March and May, 1991).
The collection of small individuals coincided
with estimated recruitment times. Small scallops
were collected from around the bases of seaweed,
arborescent polychaetes and sponges. Medium
and large individuals were more conspicuous as
they were only partly buried in the sandy sedi-
ment. They shared the substratum with poly-
chaete hummocks but were not always near the
base of emergent benthic organisms.
Differences. in recruitment from one year to
Estimated Abundance
B оо my Scallops
E] ux ur Scallops
\
Montagu Pt.
lg, Mu?
E
a lem
I
)
/ Honeymoon Boy
i
/
FIG.6. Arcas of differential abundance of the commer-
cial scallops at Honeymoon Bay in April 1992.
252
d ж
y | Maz, 9 dii
x
g
>
g
a
E!
F
g
шщ
Sues s386
a
Shell Length (mm)
FIG.7. Length-frequency histograms for Jervis Bay commercial
scallops from September 1989 to July 1991, Class-size interval
= 5mm.
another like those observed for the commercial
scallop in Jervis Bay (Table 3), are typical of
scallop species and appear to be influenced by
changes in oceanographic conditions such as
temperature and nutrient availability. Settlement
and post settlement conditions in 1989 may not
have been the same as in 1990 or 1991.
MEMOIRS OF THE QUEENSLAND MUSEUM
BIOLOGICAL STUDIES
REPRODUCTIVE CYCLE:
Gonosomatic index (GSI): Two periods
of low GSI were December 1989 to
March 1990 and November 1990 to
March 1991 (Fig.8). Four peaks occurred
in the 1989/90 cycle (August to Septem-
ber, mid-November, mid-April to mid-
May and after mid-July), and four during
the 1990/91 cycle (mid-August, late Oc-
tober, late April and mid-May). In both
years, each peak was followed by a
decrease in GSI, which may correspond
to a partial spawning event, Jacobs (1983)
described a similar situation, with at least
3 spawning peaks in Jervis Bay: late
winter to early spring, early summer and
late autumn. The reproductive cycle of P.
fumatus in Port Phillip Bay (Sause et
al.,1987a,b) showed a similar pattern with
some gamete release taking place in
winter and a major release in late spring,
Regional differences in spawning be-
haviour may exist among P. fumatus
populations in South Australia, eastern
Victoria, southern New South Wales and
Tasmania.
Parasitised gonads were present in
every sample (Fig. 9), although the fre-
quency increased from January to June
1990 and from November 1990 to March
1991. There was no obvious temporal pat-
tern in the occurrence of parasitised
gonads, but the mean frequency of occur-
rence from August 1990 to July 1990 was
higher than the similar period in the year
1989-1990, The degree of infestation
may be indicated by the orange and red in
the parasitised gonads. An orange gonad
could be the early stage of infestation in
which animals are still reproductive. A
red gonad could be a final stage of infes-
tation resulting in total loss of reproduc-
tive capacity.
Many exogenous factors influence
reproduction in scallops, but temperature
and food are most important (Macdonald
& Thompson,1986; Barber & Blake,1991). The
annual average bottom temperature (18.17°C
from August 1989 to July 1990 and 19.02°С from
August 1990 to July 1991) were significantly
different when compared in an ANOVA (df=1,
MSE=18.60, F=16.18, P>0.0001).
Temperature data were correlated with GSI and
POPULATION BIOLOGY, P. FUMATUS, JERVIS BAY
Year 1989-1990
15 Year 1990-1991
A SO ND J'F'MA'M]']J
FIG.8, Commercial Scallops gonadosomatic index
(081) from August to July in two consecutive years
in Jervis Bay.
frequency of parasitised scallops. Temperature
has been positively correlated with GSI in other
species (Paulet & Boucher,1991), but in this
study such a correlation was not clear. During the
first year, the correlation coefficients between
bottom temperature and GSI have negative
values and are not significantly different over
lags of up to 3 previous weeks. In the second year,
such correlations are not significant. The correla-
tion coefficients between bottom temperature and
the frequency of parasitised scallops are not sig-
nificant in the first year, but in the second year,
the correlation coefficients are significant over
lags of up to 3 weeks.
Bottom temperature did not seem 1o have a
direct influence on GSI, however, the positive
correlation between temperature and frequency
of parasitised scallops may be a factor that
reduces the reproductive capacity of the scallop
TABLE 3. Comparison between settlement on collec-
tors and recruitment of the commercial scallop in
Jervis Bay.
NT 1989-1990 | Recr- | 1990-1991 | Recr- |
uitment SE nitent |
199]
Ee
| Oct-Dec | peos — 79.89 oS
| [249 |
ET зз ва
| Dec-Feb | Feb
асаа Ба
(Scal/m )
253
Cw (qw 19900
e оя {үнүн
теңине of parawitised (55)
Мим
FIG.9, Percent frequency of parasitized scallops in
Jervis Bay during the sampling period from August
1989 to July 1991.
population by increasing the number of infertile
individuals. A year of low temperatures followed
by one or more of high temperatures (such as the
period August 1989 to July 1991) could reduce
recruitment in subsequent years.
Macroscopic staging: The majority of gonads in
all samples were in the Developing 2 or Spawning
1 stages (Fig.10). The Developing 2 stage made
up >40% of the samples in October and May of
both years and September and December of 19%),
The second most common stage, Spawning 1,
peaked between February and March, June,
August and November of 1990 and in March
1991, The number of ripe gonads peaked in
November 1989 and April 1991 but smaller peaks
occurred throughout the year, Spawning 2 gonads
peaked in late August of 1990, showed smaller
peaks in the previous January and June, and
showed no strong peaks in the second year of
sampling. In the first year, there was a greater
percentage of gonads in spawning condition
(Spawning 1 and Spawning 2 stages) than in the
second year. In the second year, there were more
developing gonads (Developing 1 and Develop-
ing 2 stages) and ripe gonads,
Observer expérience is needed to make correct
classifications and determine macroscopic
stages. For example, differences in gonad thick-
ness between Developing 1 and Developing 2
stages, and differences in the turgor of Ripe and
Spawning 1 gonads are determined subjectivelv.
Presence of the alimentary loop and its visual
characteristics are also subjectively determined.
The colour and outline of the loop depends on the
type and amount of food eaten and the position of
the loop within the gonad. Sometimes the loop
254
Developing 1
vn Developing 2
Ln
wa
= T
ми
т Ripe
a 400
a Stp ay:
и.
Spawning 1
Spawning 2
FIG.10. Percent frequency of macroscopic stages for com-
mercial scallops from Jervis Bay during October 1989 to
July 1991. Figures on the bottom are days of sampling.
may be pushed towards one wall of the gonad as
the gonad ripens. After almost complete spawn-
ing (Spawning 2), some gonads retain fluid so the
gut loop may be invisible.
Microscopic staging: No specimens were in the
Immature or Early Development stages as the
samples contained only adults. All other micro-
scopic stages were present most of the year
(Fig.11), Developing, ripe and resorbing scallops
occurred in 11 of 12 months with peaks in May,
April and July, and April and May respectively,
The Partial Spawning stage was present in all 12
months with a peak between February and March
and the Extended Spawning stage was found in 9
of the 12 months. Resting gonads were present
during summer (December, January, February)
and winter (June, July, August). Resting gonads
were most numerous in December and June.
The Partial Spawning stage occurred every
month, the Resorption stage occurred in all
MEMOIRS OF THE QUEENSLAND MUSEUM
months except one, and the Extended Spawn-
ing stage was not numerous. Therefore, the
commercial scallop in Jervis Bay spawns a
number of times during the breeding season.
Tt seems unlikely that complete spawning
with an entire release of gametes occurred in
Jervis Bay.
Histologic examination of gonads is im-
portant to confirm spawning, as a drop in the
gonadosomatic index (GSI) may indicate
resorption. If resorption is high, fecundity
estimates are not related to the numbers of
viable eggs released (Tremblay,1988), The
area of the gonad is also important in analysis
because a previous test (Fuentes et al., 1992)
showed that the fore region of the female part
had significantly more oocytes per follicle
than either the mid or tip regions. Further-
more, histological sections often showed an
‘edge effect’, i.c., the follicles had collapsed
at the edges of the section and the oocytes
were dislodged from the follicle walls. Con-
sequently, gonads were only compared by
taking sections from the same region of the
gonads and making classification on the
centres of the sections.
The finding that the Jervis Bay commercial
scallops have extended dribble spawning is
not unusual, In other species of scallops, ma-
ture gonads are present all year (Paulet et
al., 1988) and partial spawning or more exten-
sive spawning can occur in any month of the
year (Coe,1945), Paulet et al. (1988) sug-
gested that dribble spawning could be an
adaptation to an unpredictable environment.
The hypothesis is that at least some larvae
will find favourable conditions and the chances
of cither very weak or very strong recruitment are
minimised (Paulet et al., 1988).
Comparison of macro- and microscopic staging
schemes: Classifications derived (сот macro-
scopic and microscopic staging were compared
to determine whether a simpler technique would
allow an accurate prediction of scallop reproduc-
tive behaviour. When the macroscopic and
microscopic techniques were compared, the
macroscopic stages Developing 1, Developing 2,
Ripe, Spawning | and Spawning 2 were assumed
to correspond with microscopic stages Early
Development, Developing, Ripe, Partial Spawn-
ing and Extensive Spawning, respectively. The
comparison gave a poor result, for example
macroscopic staging consistently overestimated
the condition of developing scallops (Fig,!2),
The accuracy of the macroscopic technique was
POPULATION BIOLOGY, P. FUMATUS, JERVIS BAY
LU
do Developing
x)
E
1" ы
%
ы
3 "
4n Ripe
30
E
10 | 0
ü
ц Pattial Spawning
Extended Spawning
Relative Abundance (%)
Resorptión
Resting
а
з
19
„ y
Nov Dec Jan Feb Mar Apr May jun Jul Aud Sep Ort
FIG.11. Relative abundance (%) of microscopic stages
for commercial scallops from Jervis Bay. No
specimens were in Immature or Early Devclopment
stages.
correct only с.509% of the time when classifying
Ripe and Spawning scallops, with no consistency
between the degree of over- or underestimation
of gonad stage.
The lack of correspondence can only be partly
explained. For example: some scallops gave the
macroscopic appearance of being in Spawning 1
stage, but key histological features meant they
were placed in the microscopic Resorption stage.
]t was not possible to macroscopically classify
scallops as being in either the Resorption or Rest-
ing stage. For similar reasons, scallops appearing
to be in the macroscopic Spawning 2 stage could
have been in the Resorption or Resting stages.
Another problem may have occurred when the
loop of the alimentary canal was visible and the
gonad was therefore classified in the Developing
2 stage, but the gonad would be classified as Ripe
when viewed microscopically. When a gonad
displayed orange spots it was allocated to Spawn-
ing 1 stage. However, if the section did not en-
compass the spotted area, the gonad would be
255
25
E OVERESTIMATED
80 EQUIVALENCE
UNDERESTIMATED
CORRESPONDENCE ("53
а
Davi/ED
Day?/Dey Ripe/Ripe S 1/PS 5 2/ЕМ З
STAGE
FIG.12, Correspondence between macro- and micro-
scopic stages. DEV. 1=Developing 1; E.D.=Early
Development; DEV.2=Developing 2;
DEV .=Developing; S.]-Spawning 1; P.S.=Spawn-
ing 1; $.2=Spawning 2; EXT.S.=Extended Spawn-
ing.
classified as Ripe when viewed under the micro-
scope, The first three of the scenarios above lead
to an underestimation of gonad stage; the fourth
leads to an overestimation.
Macroscopic and microscopic staging schemes
have their own sets of advantages and disad-
vantages, Macroscopic staging is imperative
where the animals cannot be sacrificed, and its
relatively few stages are suitable for a rough
classification of gonads while in the field or under
hatchery conditions. However, the macroscopic
scheme depends very much on the observer's
ability to make correct classifications, e.g., scal-
lops classified in Spawning 1 or Spawning 2
stages might really be in Resorption or Resting
stages. The primary advantage of the microscopic
scheme is that it provides a more accurate under-
standing of an individual animal’s condition, but
a lengthy period of time is required to prepare and
process histological material. As Jervis Bay scal-
lops do not appear to have a well defined repro-
ductive cycle and they appear to dribble spawn,
provision might need to be made for microscopic
staging to follow their reproductive development
during recovery of the population.
SETTLEMENT
Two studies were aimed at the spatial and tem-
poral characteristics of scallop settlement in Jer-
vis Bay. The first study, carried out on the two
scallop beds assessed the magnitude, depth
stratification and seasonality of settlement. The
second study of 5 locations, initiated in Septem-
ber 1990 and finished in February 1991, ad-
256
Ш]
——— Murrays Beach
A Honeymearr Нау
Meso Scallpps/ Dag
Month
FIG.13. Mean and standard error of the number of
commercial scallop spat settled by month at two
locations in Jervis Bay (Oct 1989 - Oct 1990.
dressed the question of larval dispersion within
the bay and allowed comparison of settlement
between years.
In the first study, settlement data was analysed
by time, location, zone and site according to the
design described in Fuentes et al. (1992). Sig-
nificant temporal variability was found in the
TABLE 4. Summary of analysis of variance
(ANOVA) of scallop spat settlement at 2 locations, 2
zones and 3 sites over time at Jervis Bay. *0.01-
р>0.001; **0.0012p20.0001; ***p=0.0001; ns=no
significance.
[Source of variation — | ar | SS |F value | Sgnit. |
| Location [1]ил4] 532 | ns |
Zoe — — 11 1123011 1230 |. n |
Location*Zone — [1 [4850 | 220 | ms |
[Sie(LocaionZone | 6 | 13.212 | 1139 | *** |
hue — Е И 777771771 ЕГШ
[Location*Time — |s [133.110] 14.65 | we _
Zone*Time | 8 | 17.640 |
[om |
[Time*Site(Loc*Zone | 34 | 38.619 | 588 | +* |
[Lscation*Zover Time | 8 [12237] 195 | ш]
[Den — 5 0 0 [s
7.704 5.16 **
LEER [c [ss Гаа | se
ZoneDep [5] 4.323
mere — [ao fase | zs ШЕГШ
Vies kal
Location*Zone)
[Residual — — — [srz|iseoed] | — |
MEMOIRS OF THE QUEENSLAND MUSEUM
и —=— Ммитгү Бакі Ёле! — Honeyman Day Zone 1
—©— Митака Zone? = Honeymoon Bay Zone?
Mean Scallops bay,
Depth (mi
FIG.14. Mean and standard error of the number of
scallop spat settled by depth stata, zone and site.
mean number of spat that settled on the ex-
perimental longlines (Table 4). Maximum settle-
ment was recorded from November to January
(Fig.13). At both locations, settlement was mini-
mal from February to June, but increased after
July. Different temporal pattern in settlement
were observed at Murrays Beach and Honey-
moon Bay. It appeared that the duration of settle-
ment was similar at both locations, but peak
settlement seemed to occur two months earlier at
Murrays Beach. The time of maximum larval
abundance varies within P. fumatus (Young &
Martin,1989). In Tasmanian waters between
King Island and Banks Strait, similar temporal
differences in settlement were found from one
location to another, within the same location
(Young et al., 1988), and from one year to the next
(Hortle & Cropp,1987). Settlement begins in
September (Young & Martin,1989) and con-
tinues to December, but at decreasing intensities,
in southern Tasmania (Hortle & Cropp, 1987). In
eastern Bass Strait, settlement occurs November-
December (Hortle,1983; Young et al., 1988).
Differences in settlement at the first two spatial
scales (location and zone) were not significant,
but the differences among sites (within zone and
location), were significant, suggesting small
scale patchiness in settlement of P. fumatus in
Jervis Bay. Spatial variability in settlement of P.
fumatus has been documented in Port Phillip Bay;
differences were observed in the number of set-
tling spat at sites only 30km apart (Gwyther et
31,1985; Sause et 31.,1987b; Coleman, 1988).
POPULATION BIOLOGY, P, FUMATUS, JERVIS BAY
El (инт
kd [алеу
Mean scallope/bag
теа аном Нир CREE lOS I HONITMOON.
OWT mw
Location
FIG.15, Mean and standard error of the number of
scallop spat settled from December 1990 to February
1991 by location and time.
Similar variability occurs in Bass Strait in areas
separated by larges distances (Young et al., 1992),
Mean numbers of spat that settled at different
depth strata were significantly different, The
highest settlement in Zone 1 (14m) was in the
8—12m Depth strata and in Zone 2 (18m) settle-
ment was greatest at 8-14m (Fig. 14), The lowest
settlement was at the 4m depth stratum. Hortle &
Cropp (1987) found that fewer spat settled near
the surface and near the seabed (10-20m in a
depth of 31m) in Mercury Passage on the east
coast of Tasmania. Young et al. (1988,1992)
reported that larvae tended to settle on collectors
laced near the bottom rather than on those higher
in the water column off northern Tasmania, A
combination of water temperature (thermal
stratification), with factors such as larval be-
haviour, could also influence settlement in Jervis
Bay. Jervis Bay has a strong thermal stratification
most of the year (Holloway etal.,1989,1990) and
P. fumatüs settlement, like that of other species
(Mileikovsky,1973; Mann & Wolf,1983; Trem-
blay & Sinclair, 1988), may be influenced by such
stratification.
In the second study (September,1990 to
February, 1991) settlement was observed in cach
of the five locations where longlines were placed
(Fig.15). Significant spatial and temporal
variability was found in the mean number of spat
that settled on collector bags with variation oc-
curing among the five locations, among the three
times and between the two sites within each loca-
tion. Comparisons of settlement at different
depths show similarities to results of the first
study: settlement was lower on collector bags
placed near the surface than on those placed near
25]
Mean sell ps (bag
COP PONVT
MURKA yS
VLANT ANON есм
HEACH ммт
Mop MOV
WAY
масла!
FIG.16. Mean and standard error of the number of
scallop spat settled from December 1990 to February
1991 by location and site.
the bottom. Comparisons indicated higher settle-
ment at Murrays Beach and Honeymoon Bay
(outer Bay) than at the other three locations (inner
Bay). The temporal variations in this second
study were similar to those in the first i.e. highest
in Deceniber (Fig.16). However, in this year there
were no differences in the timing of settlement
between Murrays Beach and Honeymoon Bay.
It has been postulated that the larvae of P.
fumatus may nol disperse widely from the adult
population and that the number of larvae reaching
the pediveliger stage may be related to the size of
the nearby adult populations (Mason,1983;
Young et 31,1988). In Jervis Bay, there were
differences between years in the mean number of
spat that settled on collectors. More spat were
observed in December 1990 than in December
1989 at both locations, The average number that
settled pr collector was higher at Murrays Beach
probably because the abundance in the nearby
population is higher than in Honeymoon Bay.
Whether successful settlement in Jervis Bay is
related to the proximity or size of the adult
population is still to be demonstrated.
The low densities of the adult populations at
Murrays Beach and Honeymoon Bay could be the
reason for the low settlement during the study
period. The settlement figures (average of 35
scallops/collector bag in November 1989 at Mur-
rays Beach and 15 scallops/collector bag in
January 1990 at Honeymoon Bay) are less than
the average of 89 scallops/collector bag reported
in 1982 off Huskisson (Jacobs,1983). Further-
more, figures for Jervis Bay are much lower than
figures reported for eastern Tasmania (516 scal-
lops/collector bag in 1982/83, 425 scallops/col-
258
lector bag in 1984/85 and 325 scallops/collector
bag in 1985/86, Hortle & Cropp, 1987) and for
Port Phillip Bay (Sause et al., 1987b),
From these two studies, it js concluded thal
scallop larvae were distributed around the hay
and that there were limitations on settlement at
areas other than the two main beds. One implica-
lion of this conclussion is that changes which
inhibit settlement may have occurred in the
habitat at some sites, This might be the reason
why commercial scallops disappeared from areas
they were abundant in the previous decade. A
second implication derived from this study ts that
the presence of larvae in the water column, the
timing of spat settlement and the variation in
settlement with depth are relevant factors in the
design of systeins for the collection of wild spat.
The magnitude, stratification and timing of lar-
val settlement and dispersion are important
management issues for commercial scallop
fisheries, Attempts have been made to relate P,
fumatus settlement in one year to recruitment in
subsequent years [Sause et al., 1987b; Gwyther
& Burgess,1987, Coleman, 1988; Coleman &
Gwyther, 1988). Coleman (1988) found that suc-
cessful settlement may not necessarily mean
good subsequent recruitment. However, in this
study differences in settlement were evident and
they coincide with a greater adult densities
recruitment observed in the Murrays Beach and
Hone Bay beds in the transect surveys in
ACKNOWLEDGEMENTS
Thanks are due to L. Diver and A. Smith for
their technical assistance during the study and to
R. Williams for comment on the manuscript and
constant encouragement. Thanks to E. Ortiz for
assistance with the statistical analysis. Thanks
are also due to an anonymus referee and to M.
Dredge for critically reading the manuscript. |
thank the CSIRO Jervis Bay Marine Station for
providing the temperature data,
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BUTCHER, Т., MATTHEWS, J., GLAISTER, J, &
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Agriculture and Fisheries: Cronulla).
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& WEST, С. 1988. Variability in spatfall and
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M.L.C., Zacharin, W.F., & Joll, L.M., (eds),
‘Proceedings of the Australian Scallop Workshop,
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Hobart).
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YOUNG, Р.С, MCLOUGHLIN, R.J. & MARTIN,
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UNUSUALLY HIGH RECRUITMENT IN THE SHARK BAY SAUCER SCALLOP
(AMUSIUM BALLOT) FISHERY
L.M. JOLL
Joll, L,M, 1994 08 10: Unusually high recruitment in the Shark Bay saucer scallop (Amwusium
аца) Пен; Memoirs of the Queensland Museum 36(2) 261-267, Brisbane, ISSN
79-8835.
From 1983, when the Shark Bay scallop fishery reached full exploitation, to 1990, the largest
annual catch had been 731 tonnes (meat weight) in 1988. In 1991 and 1992 catches of 2,532
and 4,144 tonnes, respectively, were taken, The increased catch in 199] was attained despile
total effort being the second lowest on record, while catch per unit effort (CPUE) was the
highest on record. Total effort in 1992 increased considerably and was the highest on record,
while CPUE was second only to that achieved in 1991. The very high catches and CPUEs
were the result of a massive increase in the recruitment of juveniles derived from the 1990
breeding season. Annual stock surveys showed recruitment indices in some areas of Shark
Bay up to six times higher than the previous record. Juveniles recruited in 1990 formed the
bulk of the fishable stock in 1991, but the available effort in the fishery was unable to take
all the fishable stock in 199] and the 1992 catch was composed mostly of residual animals
carried over from the 1991 season, High recruitment into the Shark Bay saucer scallop fishery
is usually associated with low mean sea levels (2 weak Leeuwin Current) over the winter
(spawning) months. A chance association of spawning with an additional hydrological or
other environmental event favourable to larval retention or survival may have multiplied the
effect of a weak Leeuwin Current.
L.M. Joli, Bernard Bowen Fisheries Research Institute, Western Australian Marine Research
Laboratories, Р.О, Box 20, North Beach, Western Australia 6020; 9 March 1994,
The fishery for saucer scallops (Amusium bal-
loti Bernardi, 1861) in Shark Bay has been fully
exploited since 1983; its catch depends on
recruitment from the breeding season of the pre-
vious year (Joll & Caputi, in press). Surveys of
recruit (0+) and residual (1+ and older) scallops
at the start of cach season showed recruitment
from the 1990 breeding season as the highest
recorded. As a result of the very high recruitment,
the 199] catch was approximately three times
higher than the previous record catch. However.
despite the high catch in 1991, there were still
large numbers of scallops at the end of the 1991
season. These formed the basis of the 1992
fishery, which took a catch over 50% greater than
the record 1991 catch. Joll & Caputi (in press)
have shown that thesc high levels of recruitment
are associated with low mean sea levels, which
reflect periods of a weak Lecuwin Current. This
paper examines the background to the high 1990
recruitment and documents features of The even]
for use in population dynamics studies.
MATERIALS AND METHODS
Scallops are caught by vessels which are
licenced to fish only for scallops (using 25.6m
[14fm] headrope length nets of 100mm mesh)
and vessels which fish for prawns and scallops
(using (29,3m [16fm] nets of 50mm mesh)
(J01l,1987,19392), Catch and effort data are ob-
tained from a voluntary logbook system com-
pleted by all vessels in both sectors of the fishery,
and caich data are cross-checked with recetval
records of wholesale buyers. Catch per unit effort
(CPUE) data were derived from the catch and
effort of the scallop fleet. Effort of prawn/scallop
vessels may be directed at either prawns or scal-
lops or a combination of the two. To determine à
standardised effort value for the prawn/scallop
fleet equivalent to the effort of the scallop fleet,
the catch of the prawn/scallop fleet was divided
by the CPUE of the scallop fleet. Total effort was
calculated as the sum of the effort of the scallop
fleet and the standardised effort of the
prawn/scallop fleet.
Surveys of scallop abundance have been con.
ducted in November cach year since 1983, at the
end of the scallop and prawn fishing seasons and
near the end of the breeding season, using the
20m twin-rigged research vessel ‘Flinders’, A
standardised pattem of survey trawls is carried
out, using twin 50mm mesh trawls of 22,0m
(12fm) total head rope length, to determine the
abundance of scallops in the areas of the bay
where the fishery Operates. Scallops caught in
262
4000-
3000
n3
[m]
e
e
LANDINGS (tonnes meat)
©
S
87
MEMOIRS OF THE QUEENSLAND MUSEUM
30000
20000
10000
TOTAL EFFORT (hrs.)
88
YEAR
FIG. L.Scallop catchs (tonnes meat) from Shark Bay, 1983-1992 (stippled bars) and estimated total effort (+).
each survey trawl are separated, primarily on the
basis of size, into recruits (0+ animals derived
from the breeding season commenced in April
that year (Joll,1987)) and residuals (1+ and older
animals) and the numbers of each category
recorded. Most recruits are less than 75 mm in
most years and most residuals are larger than
75mm. However, when size overlaps occur, the
recruits and residuals can still be separated on the
pattern of the daily growth rings on the coloured
left valve (Joll, 1988), with recruits showing
widely spaced rings over the whole surface of the
left valve while residuals have a zone of highly
compacted rings near the valve margin.
Survey trawls are normally of 20 minutes dura-
tion and с.і nautical mile long, although in areas
of high abundance shorter trawl durations and
distances are used. The distance covered by each
survey trawl is determined either by radar fixes
(1983-1989) or from global positioning system
readings (1990 onwards) and, using the time
taken for the trawl, the average speed of the trawl
is determined. Catches of scallops are adjusted to
catches per nautical mile, while the effect of
speed on catchability is partially compensated for
by applying factors determined by Penn (unpub.)
for the catchability of adult (=residual-size) scal-
lops relative to an arbitrary speed of 3.4kt. Survey
data for each trawl shot are ultimately expressed
as the catch of scallops per nautical mile (spnm)
at 3.4kt for 12fm of head rope. The adjustment
for the effect of speed on catchability does not
compensate for the differences in catchability of
recruit and residual-sized scallops and the indices
for these two categories cannot be compared
directly. However, the data allow for year-to-year
comparisons within the categories.
Abundance indices of scallops on the main
grounds (Shark Вау N of 25° 30'S) and the
smaller ground in Denham Sound are derived
from survey data as the mean abundance of
recruits and residuals in the survey shots. The
main grounds are further sub-divided into north-
ern (N of 25° 10'S) and southern (S of 25° 10'S)
sub-areas. Because the main grounds have con-
tributed the bulk of the catch in most years, pre-
vious work (Joll & Caputi, in press) has
concentrated on the relationship between an en-
vironmental factor (Leeuwin Current strength)
and the index of abundance of recruits on the
main grounds. The survey data have also been
used to examine the relationship between index
of abundance of recruits and residuals on the main
grounds and catch from these grounds in the
following year. However, recruitment in 1990
also gave rise to a significant catch from the
HIGH RECRUITMENT IN SHARK BAY SAUCER SCALLOP
TABLE 1. Abundance indices of scallops from various
sub-areas in Shark Bay from surveys in November
1983-92 (data are spnm for 12 ftm of net towed at 3.4
kts), REC.=recruits; RES.=residuals.
|| MAINGROUNDS _| DENHAM SD. |
[1958 | sa | 73 | & | 7 | 33 |o]
[185 | i343 | 92 [227 | 2 [30 | os |
[1985 | 277 | 47 | 75 | 03 | э | 07]
[1987 | sos | 133 | 609 | as | 32 | 7 |
[ 1988 | | 2 2 |
[19 | o2 | as [ r9 | 2 | 32 | v |
[1990 | eo | 77 [356 | 73 | & | a |
[1991 | 169 | 2411 | so |4253 | 100 | saa |
[1992 | 162 | то | 157 | 467 | зві | 439 |
Denham Sound grounds and data relating
Leeuwin Current strength to recruit abundance
and catch on the Denham Sound grounds are also
presented.
Strength of the Leeuwin Current, flowing S
along the Western Australian coast during the
austral winter, is reflected in coastal sea levels
(Reid & Mantyla, 1976; Pearce & Phillips, 1988).
The major spawning activity of A. balloti in Shark
Bay occurs between April and July (Joll &
Caputi, in press) and it is the strength of the
current in these months which is most relevant,
Unfortunately, sca level data are not available for
the Shark Bay area for all of the relevant period,
but the available data shows that changes in sea
level at Carnarvon (25°, | 14" E) are reflected in
sea level data from Fremantle (32°S, 116°E) one
month later {Joll & Caputi, in press). As an index
of Leeuwin Current strength in the Shark Bay
area in April to July, the mean value of the sea
level at Fremantle, lagged опе month (1.6. May lo
August), was used,
RESULTS
CATCH AND EFFORT
Total scallop catch from Shark Bay by all ves-
sels licenced ranged from 121 tonnes ta 731
tonnes between 1983-1990, with a mean of
432tonnes/year (Fig.1). Catches for 199] and
1992 were 2,532 and 4,144 tonnes respectively.
The increase in catch in 1991 occurred despite
total effort being the second lowest on record,
while catch per unit effort (CPUE) of the scallop
fleet was the highest on record at 200.2 kg h”,
more than 5 times higher than the previous
highest CPUE. The low effort figure for the 1991
season was a consequence of processing limita-
tions on the fishing vessels, with vesscls only
fishing for a few hours each day and spending the
remainder of the day processing the catch. The
effort in the 1992 season was the highest on
record, partly as a result of a large increase in
effort hy the prawn/scallop fleet, but the CPUE
of the scallop fleet of 137.9 kg h! was second
only to the 1991 figure and nearly 4 times greater
than the previous highest CPUE recorded.
Survey Data
There was a very high abundance of recruits in
November 1990, with an exceptionally high
abundance in the southern sub-area of the main
grounds - over 6 times higher than any of the
previous indices determined for any sub-area of
Shark Bay (Table 1). Moreover, despite a high
199] catch, residual scallops in November 1991
were exceptionally abundant in the southern sub-
area and high in the other sub-areas, These high
abundances of residual scallops came about be-
cause, unlike years in which recruitment was al a
more normal level, the effort of the scallop and
prawn/scallop fleets could not fully exploit the
1991 recruitment, Another very large catch was.
taken in 1992, based primarily on the residual
scallops detected in the 1991 survey, which were
derived from the 1990 recruitment. By November
1992 the abundance of residual scallops had
decreased considerably, reflecting the impact of
the fishing activities of the fleets in the 1992
season, when recruitment to the fishery was close
tọ normal.
Recruitment levels for 1990, categorized into 3
abundance classes (Fig. 2А), show that recruit-
ment was not uniform through the sub-areas. The
principal area of recruitment was in the southern
sub-arca of the main grounds, with a core of very
high abundance (5,000 spnm) surrounded by an
area of relatively high abundance (1,000-4,999
spnm). Residual scallop abundance in the 1991
survey (Fig. 2B) reflected fairly closely the dis-
tribution of recruits in the 1990 survey, with the
exception of a small area of high abundance of
residual scallops in the northem arca. The ap-
parent emergence of this area of residual scallops
in November 1992 may indicate that recruitment
in the northern sub-area occurred slightly later
and was not measured fully by the November
199] survey,
The highest recorded catches (12fm net at
3.4kts) in the 1990 and 1991 surveys were 19,075
264
10'
20'
30'
40'
113? ш 20 307 40!
Number / nautical mile:
* 0-999
e 1000—4999 6 5000 +
MEMOIRS OF THE QUEENSLAND MUSEUM
Number / nautical mile:
0-999 œ 1000—4999 65000 +
FIG. 2. Distribution of the abundance of scallops in Shark Bay recorded in surveys in November.
A, Recruits, 1990. B, Residuals,1991.
recruit spnm in 1990 and 59,242 residual spnm in
1991. Conversion of these catch rates to abundan-
ces requires application of catchability factors
relevant to the various trawl speeds. Joll & Penn
(1990) showed that the catchability of residual-
size scallops at a speed of 2.5 kt is approximately
0.6, while the adjustment of this catchability fac-
tor to the higher trawl speed of 3.4kt at which the
survey data are expressed is also 0.6 (Penn un-
publ. data). Application of these catchability fac-
tors to the 1991 figure for the highest catch of
residual scallops equates to an actual abundance
of 164,561 spnm in the path of the trawl. Assum-
ing a trawl path of 60% of the headrope length of
trawl used (Joll & Penn,1990), the area swept in
a 1 nautical mile trawl would be 24,446 m*. On
this basis density of adult scallops in the shot with
the highest abundance of residual scallops in the
1991 survey was estimated to be 6.7 scallops тг.
The catchability of recruit scallops has not yet
been formally determined. However, based on
the lower swimming capacities of smaller scal-
lops and the increased latency of their response
to a stimulus to swim (Joll 1989b), it could be
expected that the catchability of recruit-sized (50-
60mm) scallops would be about 30-40% of that
of residual-size scallops. On the basis that the
catchability of recruit-size scallops is 40% of that
HIGH RECRUITMENT IN SHARK BAY SAUCER SCALLOP
77 78 79 80 81 82 яз B4 B5 86 87 BB
SEA LEVEL (MAY — AUGUST)
73 74 75 76 77 78 79 80 ві 82 вз ва B5 86 87 вв
SEA. LEVEL (MAY — AUGUST)
FIG. 3. Relationship between the index of recruit abundance (mean spnm) in Shark Bay and mean Fremantle sea
level (cm) over the period May to August. A (bottom left), Main grounds. B, (upper right), Denham Sound.
of residual-size scallops, the density of recruit
scallops in the path of the trawl with the highest
catch of recruits in the 1991 survey was estimated
at 5.4 scallops m”.
EFFECTS OF THE LEEUWIN CURRENT
The relationship between the recruitment index
for the main grounds and mean Fremantle sea
level over the period May to August in the period
1983 to 1992 (Fig. 3A) showsthat the recruitment
index was highest in years when the mean sea
level was low. Recruitment data for Denham
Sound (Fig. 3B) show a high recruitment index in
1990, when sea level was low. However, in 1987
when sea level was similarly low, there was no
increase in recruitment and in 1992, when sea
level was not particularly low, there was also a
high recruitment.
In most years there is a high level of exploita-
tion of the scallops recruiting to the fishery,
which gives rise to a strong correlation between
the abundance index of recruits in one year and
catch on the main grounds in the following year
and between sea-level in one year and catch on
the main grounds the following year (Joll &
Caputi, in press). However, because of the in-
ability of the fleet to fully exploit the fishable
stock available in 1991, there was a considerable
carry-over of scallops into 1992. This gave rise to
a high catch in 1992 on both the main grounds
and the Denham Sound grounds which was not
related to the sea- level of the immediately pre-
vious year (Fig. 4A,B). Although survey data
showing recruitment strengths are not available
prior to 1983, the inclusion of data for the 1983
catch and the 1982 sea level provide additional
confirmation of a relationship between sea level
and catch in the following year (except 1992) on
both the main grounds and in Denham Sound.
DISCUSSION
The high 1990 recruitment led to massive 1991
and 1992 scallop catches. On a live weight basis,
the 1992 catch of scallops was over 20,000 ton-
nes, making it the second largest single species
fishery in Australia in that year after greenback
jack mackerel. The combined catch of scallops by
the scallop and prawn/scallop fleets in 1991 and
1992 was a little less than twice the accumulated
catch of the previous eight years. The mean
CPUE in 1991 was approximately 18 times
greater than the highest mean CPUE in the
Queensland saucer scallop fishery 1976-1987
(Dredge, 1988). Using an estimated mean adduc-
$
8 E EBf
CATCH (tonnes)
500
75 76
73 74
* 90/91
350
"91/92
— 300
:
250 :
8 82/83 B
— 200
B шо
100
50
D ^ 87/88 ^ 86/87 ^ 85/86
73
74
tor weight of 15g for scallops caught in 1991 and
a mean of 20g for 1992, there were approximately
376 million scallops caught over 2 years. If most
of these scallops had been caught in the year in
which they recruited to the fishery, as is the case
in years of more normal levels of recruitment (and
without adjusting for the natural mortality of scal-
lops from 1991 to 1992), an estimated catch of at
least 5,500tonnes would have been taken in 1991.
Estimated densities of scallops in the area of
highest abundance in the 1990 and 1991 surveys
were as high as 6.7 scallops m. Higher densities
of residual scallops than recruit scallops is
probably a reflection of local variation in abun-
dance, with slightly different areas trawled in
77 78 78 во в 82 аз 84
SEA LEVEL (MAY — AUGUST)
75 76 77 78 79 BO Bl 82 BS 84 85 88 87 88
SEA LEVEL (MAY — AUGUST)
FIG. 4. Relationship between total catch (tonnes meat) and mean Fremantle sea,
level (cm) over the period May to August in the previous year. (86/87): (Year
of sea level data/ Year of catch data). A (upper), Main grounds. B (lower),
Denham Sound ( open circles represent ycars of no effort in Denham Sound).
MEMOIRS OF THE QUEENSLAND MUSEUM
different years. Alterna-
tively, differences in catch-
ability between recruit and
residual-size scallops may
have been underestimated,
which would reduce the es-
timated density of recruits.
Based on the minimum
value of the upper category
of abundances (Fig.2), this
figure equates (on the same
basis as previous estimates)
to densities of 0.57 residual
scallops m? and 1.42 re-
cruit scallops т>. Dredge
(1988) noted maximum
density of Amusium balloti
in Queensland at around
Im~, while Joll & Penn
(1990) reported densities of
scallops of 0.08-0.09m* in
an area of Shark Bay in
1986. The area occupied by
scallops at an estimated
average minimum density
of 1.42 recruits m^ and
0.57 residuals m? in the
1990 and 199] surveys
(based on the area enclosed
85 B6 87 88
суи by shots of 5,000 spnm or
greater) was around 90km?
* 89/90 and 60km? in the two years
в8/89°° 84/85
respectively. Scallops at
very high local densities
and generally high den-
sities across a wide area
over a two year period indi-
cate a capacity of the en-
vironment to support large
numbers of scallops
without any apparent
depletion of the food resources.
The mechanism by which the Leeuwin Current
affects recruitment success in Shark Bay is not
fully understood. Satellite imagery shows that,
when the Leeuwin Current is near the coast off
Shark Bay, bodies of warm Leeuwin Current
water sometimes move away from the main flow
of the current and enter Shark Bay (Joll & Caputi,
in press). This may flush larvae out of the bay or
into the saline embayments along the mainland
shore line. Hydrological flushing has been recog-
nised as factor affecting recruitment in Placopec-
ten magellanicus and Pecten maximus
(Dickie,1955; Caddy,1979,1989; Thouzeau &
HIGH RECRUITMENT IN SHARK BAY SAUCER SCALLOP
Lehay, 1988). Alternatively, the higher tempera-
ture ог lower nutnent levels of the Leeuwin Cur-
rent water (Pearce,1991) may provide an
environment which is less suitahle for larvae.
The abundance pattern of 1990 (Fig. 2А) had a
core of very high abundance surrounded by an
area of relatively high abundance, suggesting thal
larvae were contained within a well-defined eddy
feature at settlement, Dredge (1988) suggested
that a gyre in Hervey Bay in Queensland may act
to trap larvae of А. ballot and Caddy (1979)
hypothesized that recruitment to the Bay of
Fundy scallop fishery was positively influenced
by the degree of retention of larvae within a gyre.
Greater retention of larvae within Shark Bay,
perhaps inside clearly defined hydrographic fea-
lures, appears to be the most likely cause of
incre: larval survival and juvenile recruitment
in years when the Leeuwin Current is weak,
The reason for disproportionately high recruit-
ment in 1990, compared with that occurring at
similar average sea level values in 1982 and 1987
isnotclear. There may have been some additional
hydrological factor which led to ап unusually
high level of larval retention within the bay or
some other environmenial event which led to à
high retention or survival rate of larvae. The
action of an additional fayourable factor or fac-
tors Within a low sea level environment already
basically conducive to good larval survival may
have been greatly heightened by the synchronisa-
tion of that event with spawning activity,
Amusium balloti is à multiple spawner (Joli,
1987,1989a) with a larval life of c.22 days (Rose
et al., 1988). With a relatively short larval life it
may be that recruitment success can bencfit from
chance associations between spawning and short
term environmental factors which are not
reflected in the mean sea level, The very compact
and unimodal size-frequency distribution of the
1990 recruits in the arcas of very high abundance
suggests that the bulk of the recruits in these areas
were derived from one spawning, Synchranisa-
tion of an additional hydrological or other en-
vironmental event with а period of spawning may
have led to the levels of recruitment success
observed in 1990,
LITERATURE CITED
CADDY, J.F. 1979. Long-term trends and evidence for
production cycles in the Bay of Fundy seallop
fishery. Rapports et Procés- Verbaux des
Réunions de Conseil International pour L-
Exploration de la Mer 175: 97-108
CADDY, LF. 1989, Recent developments in resennch
267
and management for wild stocks of bivalves and
gastropods. Pp. 665—700. In J.F, Caddy, (ed),
'Marine invertebrate fisheries: their assessment
and management’. (John Wiley and Sons: New
Yor!
)
DICKIE, LM, 1955. Fluctuations in abundance of the
giant scallop, Placopecten magellanicus
(Gmelin), in the Digby area of the Bay of Fundy,
Journal of Fisheries Research Board Canada 12:
797-857,
DREDGE, М.С. 1988 . Recruitment overfishing in а
tropical scallop fishery. Journal of Shellfish Re-
search 7; 233-239,
JOLL. L.M. 1987. The Shark Bay scallop fishery,
Fisheries Department Western Australia,
Fisheries Management Paper 11:1—123.
JOLL, LM, 1988, Daily growth rings in juvenile saucer
scallops, Amusium ballon (Bernardi). Journal of
Shellfish Research 7: 73-76,
JOLL, L.M, 19892. History, biology and management
of Western Australian stocks of the saucer scallop
Amusitm ballati. Pp. 30-41. In M.L.C. Dredge,
W.F. Zacharin & L.M, Joll, (eds), ‘Proceedings of
the Australasian Scallop Workshop, Hobart’.
(Tasmanian Governnient Printer: Hobart),
JOLL, L.M, 1989b, Swimming behaviour of the saucer
scallop Amusium ballot’ (Mollusca: Pectinidae).
Marine Biology 102: 299-305.
JOLL, L.M, & САРТ, N, in press, Environmental
influences on recruitment in the saucer scallop
(Amusium balloti) fishery of Shark Bay. In
‘Procecdings of the Shelliish life histories and
shell fishery models Symposium’. ICES Marine
Sciences Symposia.
JOLL, L.M. & PENN, J.W. 1990, The application of
high- resolution navigation systems to Leslie -De-
Lury depletion experiments for the measurement
of trawl efficiency under open-sea conditions.
Fisheries Research 9: 41—55,
PEARCE, А Р. & PHILLIPS, B.F. 1988. ENSO events,
the Leeuwin Current, and larval recruitment of the
western rack Jobster, Journal du Conseil (Interna-
tional pour L'Exploration de la Mer) 45:13-21.
PEARCE, А.Е. 1991, Eastern boundary currents of the
southern hemisphere. Journal of the Royal Society
of Western Australia 74: 35-45.
REID, J.L. & MANTYLA, A.W. 1976, The effect of
the geostrophic flaw upon coastal sea elevations
in the northern. North Pacific Ocean, Journal of
Geophysical Research 81:3 100-3) 10.
ROSE, К.А. CAMPBELL, С.К, & SANDERS, 5,6,
1988. Larval development of the saucer scallop
Amusium balloti (Bernardi) (Mollusca: Pec-
tinidac), Australian Journal of Marine and Fresh-
water Research 39: 153—160.
THOUZEAU, G. & LEHAY, D. 1988. Variabilité
spatio-temporelle de la distribution, de là crois-
sance et de 1а survie des juvéniles de Pecren
mavinuis (L.) issus des poates 1985, en baie de
Saint-Brieuc. Oceanologica Acta 11; 267-283,
HATCHERY PRODUCTION OF WESTERN AUSTRALIAN SCALLOPS
DEREK A. CROPP
Cropp, D.A. 1994 08 10. Hatchery production of Western Australian seallops. Memoirs of
the Queensland Museum 36(2): 269-275. Brisbane. ISSN 0079-8835.
Adult scallops (Amusium balloti, Chlamys australis and Chlamys scabricostata) from Shark
Bay, Western Australia were transported to 6,0001 and 12,0001 pools of raw seawater al a
commercial hatchery. Adults were fed daily with cultured microalgae to improve gonad
condition, Successful, induced spawnings were conducted for all species with A. balloti
spawnings conducted in all months from April to December inclusive, over a 3 year period.
Adults were induced to spawn by the addition of sperm and a water temperature increase.
For the most successful batches, larvae were respectively reared to settlement in 12, 12 and
17 days al 22,.2+0,94°C, 24.35 * 1,2°C and 20,2 +0.8° C. Up to 6.1 million pediveligers were
placed into settling tanks from one spawning, Batches of settled spat regularly exceeded 0.5
million with the highest count attained of approximately 2.4 million spat at the completion
of the metamorphosis/settlement stage, Large scale hatchery production techniques were
developed and a potential for aquaculture has been shown, particularly for C. australis.
Derek A, Cropp, Aquatech Australia Pty Lid, 15 Wignall Street, North Hobart, Tasmania
7000; 9 March 1994.
Ten species of scallops have been documented
in W.A. waters (Wells & Bryce,1985) but an
accurate number or species is difficult to obtain
duc (0 species overlap and name changes.
Of commercially important W.A. species, on]
Amusium pleuronectes australiae {Наће,1964)
was omitted from Wells & Bryce’ s list; it was also
omitted from a record of scallops in Shark Bay
(Slack-Smith, 1990). Jt is occasionally part of the
by-catch of the Amusium balloti (Bernardi, 1861)
fishery and forms the basis of a small fishery in
the Northern Territory (Young & Martin, 1989).
Other commercial species are Pecten modestus
(Reeve, 1852), Chlamys australis (Sowerby,
1842), Chlamys (Mimachlamys) asperrimus
(Lamarck,1819), Annachlamys leopardus
(Reeve, 1853) and Amusium balleri. Pecten
fumatus Reeve.1852 may occur along the
southern coast of W. A. (Joll, 1988) but hatchery
production of it is documented (Cropp,1988a;
Cropp & Frankish, 1988; Dix & Sjardin,1975). A.
balloti and A, pleuronectes australiae belong to
the Amusiidae; other species mentioned above
bclong to the Pectinidae.
Of recent studies on hatchery production of
Australian scallops (Connolly,1990; Cropp,
19882, Cropp & Frankish, 1988; Dix & Sjardin,
1975; Rose ct al.,1988; Rose & Dix,1984) only
one (Rose & Dix,1984) dealt with Chlamys as its
commercial importance in Australia has been
minimal; two (Connolly, 1990; Rose et al., 1988)
reviewed hatchery culture trials on A. ballori.
Rose & Dix (1984) provided information on
larvae of the doughboy scallop, Chlamys asper-
rimus, Which is similar to C. australis from W.A.
They occupy similar ecological niches but the
temperature regimes of their environs are 9-20°C
for C. asperrimus and 17-25°C for C. australis
(Cropp, 1993h)
A. balloti, the saucer or swimming scallop, is
the target species for significant trawl fisheries in
central Queensland (Williams & Dredge.1981)
and Shark Bay, W.A, (Jo1], 1987). It is a tropical-
subtropical species which appears to prefer 19-
24°C water on medium to coarse sandy mud
bottoms. Its natural spat settlement and recruit-
ment have been studied (McDuff, 1975; Kettle,
1984; Dredge,1981; Campbell, 1987; Sumpton et
al.,1990) as has hatchery culturing (Rose et al.,
1988; Connolly,1990), These studies were
hindered by the tendency of the metamorphosing
larvae not to exude a strong byssal thread (Rose
ct al.,1988; Dredge,1981). The attachment was
also found to be for a short time period only (Rose
et a1.,1988), unlike Pecten or Chlamys (Dix &
Sjardin,1975; Rose & Dix,1984; Sause et al.,
1987; Hortle & Cropp, 1987; Cropp,19933), Im-
provements in broodstock conditioning and lar-
val rearing techniques (Gwyther et аї.,1991;
Cropp.1988a) have been implemented and fur-
ther developed in the study reviewed herein.
In Shark Bay the scallop by-catch of the A.
ballati fishery is c.1-5% C. australis, C.
scabricostata and A. leopardus (plus occassional
rarer specics). This by-catch is generally returned
lo the sea as processing is deemed difficult and
markets have not been established. However, the
meat and gonw! from processed C. australis 18
270
almost identical to that from the cooler water C.
asperrimus, which is common in Tasmania and
well reccived on the market. Hence a potential
exists for marketing C. australis, Chlamys
scabricostata does not grow to the same adult size
that C. australis or C. asperrimus does (S. Slack-
Smith pers. comm.). C. australis was deemed as
having a better potential for aquaculture,
The most common spat production technique
for overseas scallop culture is based around col-
lection of natural spat at sea (Ito,1988; Bull,
1988). From this perspective alone, it is necessary
to be able to distinguish between larvae which are
likely to be present in the water column al similar
times, For this reason а small scale hatchery trial
involving C. scabricostata was conducted prior
to the culture of C. australis. Adults of this
species were induced to spawn and the larvae
reared under the same conditions as for C.
australis. Larvae produced by A. ballot adults
have been reared under similar conditions and are
distinguishable from C. australis and C.
scabricostala.
Various Chlamys species are cultured in
hatcheries and in some areas grown-out in culture
operations overseas (Broom & Mason,1978;
Mason, 1983; Cropp,1988h). Chlamys generally
attach firmly to substrates upon settlement and
remain attached for several months. Interception
of the natural settlement and spat attachment
process (with artificial substrates) has been found
to be viable (Hortle & Cropp.1987) and economi-
cally feasible on a large scale (Rhodes & Wid-
man,1980; Маги,1985; Сгорр,1987) for namer-
ous different species overseas and within
Australia (Bull,1988; Cropp,1988b). It has al-
lowed industries to develop through the
availability of large amounts of spat.
Amusium, however, exhibits a weak and tem-
porary attachment (byssus) only (Dredge, 1981;
Gwyther et al.,1991) and collection of significant
quantities of spat at sca is therefore unlikely. This
necessitates the hatchery production of spat, The
small population of C, australis in Shark Bav,
suggests that natural spatfall would probahly he
minimal and thus, if an aquaculture industry was
to develop for this species, hatchery culture
would also be necessary, Significant quantities of
C. scabricostata would probably be obtainable
from spat collectors deployed in Shark Bay,
hence the hatchery trial simply examined larval
development in this species.
An assessment of meat recovery (as % of live
weight) from various species suggested thal most
market potential was in A. ballot! and C.
MEMOIRS OF THE QUEENSLAND MUSEUM
australis. Larval development of these species
was carefully examined in addition to a brief
larval rearing trial with C. xcabricostata. The
southern species, P. modestus is acknowledged to
have aquaculture potential also. The fact that it is
similar to P. fumatus, which has been produced
in commercial quantities in a hatchery, suggests
that the larval rearing techniques for both species
would probably be similar, Thus no special effort
was made to rear the species іп a hatchery, As an
aquaculture species, A. leopardus was perceived
to be inferior due to slow growth and relatively
low meat recovery from live weight; consequent-
ly ho hatchery work was conducted on this
species.
MATERIALS AND METHODS
Saucer scallops were collected from trawlers in
Shark Bay, between April and October of each
year from 1989 to 1991. Broodstock for the C.
scabricostata spawning were collected in June
1990, and for the C. australis spawning in July
1991, The scallops were obtained from sorting
trays, placed into either small portable tanks con-
taining aerated water or into steel mesh baskets
in the vessels’ circulating tanks. Scallops held in
the vessels tanks were placed into small portable
tanks upon arrival in port. They were transported
in the tanks, hy road, to a hatchery at Camarvon
and placed in 60001 and 12000] above-ground
swimming pools, at c.15-20°C, for 5 days prior
10 а spawning being attempted with some of the
animals. In mid-winter, 2 kW clectrical immer-
sion heaters were used to maintain the water
temperature.
Saltwater was pumped through cartridge filters
in the series: 20pm, 10pm, Sym, 24m and lum.
Broodstock pools were filled with 20m filtered
water, larvae tanks with 1—204.m filtered water,
depending on the daily water quality (thorough
filtering for dirty water), and Im filtered water
was used for algal cultures.
During the broodstock holding period, 50% of
the pool volume was changed at least every
second day and on occasions daily. Initially,
volumes of a non-axenic algal culture, Terraselt-
mis suecica, were added daily in sufficient quan-
tity to establish a food cell density in the holding
pool of 30000—40000 cells mi’. After early
gonad conditioning work exhibited poor results.
the algae species was changed to another non-
axenic alga, Chaetoceros gracilis. When avail-
able, this diet was supplemented (with
approximately 5000 cells тї!) by non-axenic
HATCHERY PRODUCTION OF WA SCALLOPS 271
TABLE 1, Average annual results for each of the development stages per spawning batch of A. balloti.
eat но. ot Bashes Не Females! E | D. | —P | Peseta Size (uri) |“ Doves settee |
ТЛ | 1326 | 27.4005 | 1.4298 | 68658 | — 20245 | isa |
i ou» uem ewe [ене ша [ци —
Hen E eee ee ee чар
F= No. Fertilized eggs (x 10°); D= No. 'D' veligers (x 105; P= No. Pediveligers (Сгорр,1993а)
TABLE 2. Average annual size and growth rates of A, balloti larvae per batch. Only batches where an accurate
Day 2 and pediveliger size were available are documented in this table, hence the batch and pediveliger
difference to Table 1. Pediveliger size is taken as the larval size on settlement day (Cropp, 19932).
Size of Г” e
day 2(
TIR
119.70
Po
Batches
1989| 10
Chaetoceros calcitrans, Pavlava lutheri and
Tahitian Jsochrysis (aff.) galbana.
Gonad condition of live scallops was monitored
visually on a regular basis. When well developed
or mature gonads were apparent, selected animals
were cleaned and a spawning was attempted. In
most spawnings, 4—10 male and 10—20 female A.
balloti were used. For the other spawnings, 3
male and 6 female C. australis, and 2 male with
4 female C. scabricostata were used. A combina-
tion of water temperature increases and the addi-
tion of sperm extracted from spare broodstock
were used as spawning stimuli.
RESULTS
Thirtyfive successful spawnings of A. balloti
were conducted over the 3 years (Cropp,
1993a)(Tables 1,2). A maximum of about 6 mil-
lion and often in excess of 3 million eggs were
obtained from A. balloti females, up to 4.5 million
eggs from C. australis females and about 0.5
million eggs per C. scabricostata female resulted
from induced spawnings (Сгорр, 19936).
Larvae were reared in larvae tanks at a salinity
of 35ppt and an average temperature of 22.2+
0.94*C (mean s.d.) for А. balloti, 24.35+1,2°C
for C. australis and 20.2--0.8"C for C. scabricos-
tata. The algal diet was composed of similar
portions of C. calcitrans, P. lutheri and Tahitian
Г. (aff.) galbana at a density increasing from
10000 cells тЇ`! on day 2 up to 15000 cells mr’
at day 12 (settlement) and then to 25000 cells ml"!
Pediveliger size (jum) Daily growth of larvae, dav
pem 2 to settlement RENE A m day”
214.38 14.30
123.30
| | 20738 | 78 | 1540 |
746 |
for settled spat. The diet of one batch of A. ballot
larvae reared in July-August 1991 (Fig.1) is rep-
resentative of that fed to other larval batches.
Larval water was changed totally on or about
every two days. For the most successful batches,
larvae were respectively reared to settlement in
12, 12 and 17 days (Cropp,1993a,b).
Larval development for A. balloti commenced
with eggs of 75.9+4.4pm (n=40), a first ‘D’
stage veliger (day 2) of 123.3+2.06.m and a
pediveliger of 211.0 1.41,um (Fig.2). Larval
development of A. balloti as documented (Rose
et 31,1988) was verified in this work. For the
batch being examined, 4.1 million pediveligers
were put into settlement tanks with 30 mesh spat
BE Cealcitrans
Ш Puthen
Bl sachrysis
Celle/ larvae
FIG. 1. Algal diet for A. balloti larvae during the culture
phase, July-August 199].
272
700.
S ою
=
2 500.
© "
$ «o 9
i
300. z
ў
100, Larvar
FIG, 2. Growth of A. ballori larvae and spat, July-
August 1991 (Cropp,1993a).
collectors. On day 19 spat were 362 jum in size,
and by day 28, 1.4 million spat of 772 jum in size
were present (41,333+4,509 spavcollector plus
160,000 loose spat). The shell of spat gradually
changed from opaque to white as they grew
(74mm), a feature which has not been docu-
mented previously. It is also an aspect to be
considered when identifying naturally occurring
spat collected in tropical and sub-tropical areas.
Mature eggs of C. australis were 62,2+2.2pm
(n =30) and the first D-shaped larvae were
108.5+4.1 um long (Fig.4). Total eggs produced
from the 4 females was 12.55 million (Fig.5). By
day 4 the larvae measured 124.1 *-5.0j.m long.
The D- shaped larvae developed rapidly up to day
8 when a characteristic scallop larval shape was
displayed.
E
Egis
b 8
Number of Eggs, Larvar, Spat [x 108]
B
Eetllement
FIG. 3. Number of A. balloti eggs, larvae and spat
surviving during the culture phase, July-August 1991
(Cropp,19933).
MEMOIRS OF THE QUEENSLAND MUSEUM
Precise size of fully developed pediveligers is
difficult to ascertain; for C. australis, a sample of
swimming pediveligers taken on day 12 had a
mean length of 203.6+12.1рт, At that stage
c.50% of the larvae had an motile foot. A sample
of settled spat on day 15 had a length of
296.9+48.3 jum. Thin new (dissoconch) shell
was evident on the outer edge of spat. About 2,4
million spat (39.3%) settled from 6.1 million eyed
larvae at day 12. The spat count included an
estimation of those spat attached to the wall and
bottom of the tank. A sample of 5 collectors was
washed and spat counted on day 16, The mean
count per collector was 37,8002-6,058; spat were
approximately 300j.m.
Eggs produced by C. scabricostata females had
a diameter of 60—63,.m (Fig.6). Sufficient sperm
Mean shell Jeagth (pm)
Settlement
Day
FIG, 4. Growth of C. australis larvae and spat: (shell
length with s.d.); (n = 30) (Cropp,1993b).
solution was added to give a ratio of 4—5 sperm
per egg (with a total of 1.54 million eggs, Fig.7).
After 46 hours (day 2) at 21.8°C, 800,000 larvae
(51.95% of eggs) had developed into D-shaped
veligers with a mean length of 103.4рт (Figs
6,7) and 82.2jm height. At day 13 the larvae
were 197pm long and exhibited a prominent
eye-spot. By day 17 numerous pediveligers were
evident. The 75,000 remaining larvae were
220um long and had grown at a rate of &.33рт
day ! since becoming D-shaped larvae at day 2.
Metamorphosis and settlement. occurred over
days 17-20. By day 21 settled spat were 250m
long and exhibited new shell.
DISCUSSION
Examination of the commonly assessed phases
HATCHERY PRODUCTION OF WA SCALLOPS
Number of Eggs and Larvae (106)
Settlement
Day
FIG. 5. Number of C. australis eggs and larvae surviv-
ing during the culture phase (Cropp.1993b).
of the larval rearing stage for А. ballorí indicated
that culture in 1991 was markedly more success-
ful than culture of larvae in 1990 and especially
1989. A combination of factors was responsible
for this success. The major improvements were
in broodstock conditioning, and thus quality of
eggs, the effectiveness of the water filtration sys-
tem (improved water quality) and variations in
the larval culture conditions (Cropp. 19932).
Use of high quality mature eggs produced
benefits throughout the larval culture period. Less
mortality occurred and larger veligers resulted.
Subsequent use of water with a stable tempera-
ture and salinity further enhanced the growth and
survival of larvae.
An average survival figure in 1991 of 14,8%
from eggs to D-shaped larvae and 64.4% from
È
B
ы
Mean shell length (yn)
З
Seulement
FIG. 6. Growth of С, scabricostata larvae and spat:
(shell length).
D-shaped larvae to pediveligers compares
favourably with data from Canadian research
(Thompson et al.,1985) on the Japanese scallop
Patinopecten yessoensis. Larval rearing of this
species produced survival rates for corresponding
phases of 1096 and 1096. These figures may have
resulted from the use of antibiotics and non-
axenic algae in the culture process.
Rose et al. (1988) recorded a growth of 5.21.m
day"! for A. balloti larvae from the first D stage to
the umbonal veliger, then 6.3m day” until the
pediveliger stage. Larvae in our study attained àn
overall average (for 1991) of 7.5j.m day” for the
period from the first D-shaped larvae (day 2) to
pediveliger. The batch spawned on 24 July 1991
gave an overall growth rate of 8.7m day" for the
same phase.
Rose & Dix (1984) found that the mean egg
Spel
Number of Eggs, Laryae, Spat x 106}
Settlement
FIG, 7. Number of eggs, larvae and spat for C.
scabricostata during the culture phase.
diameter for C. asperrimus was 61.5 +0.4рт, the
first D-shaped larval stage with a prodissoconch
I shell occurred after 2 days and was 108 рт long,
and that fully developed pediveligers occurred on
day 19, when larvae were 194рт long. Cor-
responding data for C. australis were 62.2+ 2.2
jum, 108.5+4.1 jum and 203.6 12.1 рт (day 12)
respectively.
C. australis larvae were reared at 23-24°C іп a
subtropical area, whilst C. asperrimus larvae
(Rose & Dix,1984) were reared at 17-18?C in a
cool temperate area. The higher rearing tempera-
ture for C. australis is thought to be responsible
for the comparatively short larval period.
Larval development appears to be very similar
for C. asperrimus and. C. australis and the spat
settle at a similar size. In the C. australis trial,
63.756 of eggs developed into D-shaped larvae,
274
76.3% developed from D-shaped larvae to
metamorphosis and overall, 43.6% of eggs
developed through to metamorphosis. These are
extremely high survival rates and as far as known,
they exceed those documented for hatchery cul-
ture of any other species of scallop world-wide.
Overall, these trials have established viable
techniques for the production of commercial
quantities of А. balloti and C. australis. For A.
balloti this may mean that large quantities of spat
could be used to enhance the wild fishery, al-
though this is unlikely to be required at present
due to the buoyant state of the fishery, Catches
recorded recently in W.A. have been higher than
previous peaks in the history of the fishery. As-
socialed declines in market value, and a
depressed world scallop market, have threalened
the commercial viability of the scallop trawling
industry and eliminated the possibility of a viable
culture industry at present. For C. australis,
hatchery produced spat may allow its aquaculture
potential to be developed as it has been for C.
asperrimus in Tasmania. However, the economic
value of C. australis would be affected by the
depressed market and commercial viability of a
culture operation would need careful examina-
tion before proceeding.
ACKNOWLEDGEMENT
This research was funded by the Fishing In-
dustry Research and Development Council.
LITERATURE CITED
BROOM, MJ, & MASON, 1. 1978, Growth and spawn-
ing in the Pectinid Сату opercularis in relation
to jure and phytoplankton concentration.
Marine Biology 47: 277-285.
BULL, M.F. 1988. A New Zealand scallop enhance-
ment project-cost and benefits. Рр, 154—165, In
M.C.L. Dredge, W.F. Zacharin & M. Joll (eds),
‘Proceedings ofthe Australian Scallop Workshop,
Hobart, Australia’. (Tasmanian Government
Primer: Hobart),
CAMPBELL, С.К. 1987, ‘A final report to the Fishing
Industry Research Committee on the recruitment
into commercial stocks of the saucer scallop
Amusium japonicum balloti,’ 27p.(Queensland
Department of Primary Industries: Deception
Bay)
CONNOLLY, N.M. 1990. Some aspects of the maricul-
ture of the tropical/subtropical scallop, Aemusium
balloti (Bernardi). Hons Thesis, James Cook
Univ. North Queensland, 93
CROPP, D.A. 1988a. 'FIRTA АЗ Hatchery produc-
tion of scallop spat for large scale reseeding trials.
Final Report’. (Fishing Industry Rescarch Trust
MEMOIRS OF THE QUEENSLAND MUSEUM
Account, Department of Primary Industry: Can-
berra
CROPP, D.A. 1988b. Scallop culture in the Pacific
Re. Region, Pp. 193-211, In Evans, L.H. & O'-
ivan, D.B., (eds), ‘Proceedings of the First
jim s Shellfish Aquaculture Conference
Perth" (Curtin University of Technology; Perth).
CROPP, D.A. 1992, ‘Aquaculture of the saucer scallop
Amusium balloti, Final Report’ (Fisheries Re-
search and Development Corporation, Depart-
ment of Primary Industries: Canberra),
CROPP, D.A, 1993a. Development of large scale
hatchery production techniques for Amusium bal-
юй (Bernardi, 1861) in Western Australia.
Aquaculture 115: 285-296
CROPP, D.A, 1993b, Hatchery culture potential of the
scallop Chlamys australis in Western Australia.
Aquaculture 115:31-40
CROPP, D.A. & FRANKISH, KR. 1988. Cost coin-
parion of hatchery and naturally produced spat
or the scallop Pecten fiwnatus Reeve. Pp. 196-
225, In M,C,L. Dredge, W.F. Zachann & L.M.
Joll, (cds), ‘Proceedings of the Australian Scallop
Workshop, Hobart’ (Tasmanian Government
Printer: Hobart),
CROPP, R.A. 1987, Feasibility of scallop culture in
Tasmania, Tasmanian Department of Sea
Fisheries Technical Report 15: 1-24,
DIX, T.G. & SJARDIN, M.J. 1975, Larvae of the
commercial scallop, Pecten meridionalis From
Tasmania. Australian Journal of Marinc and
Freshwater Research 26; 109-112.
DREDGE, M.C.L. 1981, Reproductive biology of the
saucer scallop Amusium japonicum ballor (Ber-
nardi) in central Queensland waters. Australian
Journal of Marine and Freshwater Research 32:
775-787
GWYTHER, D., CROPP, D.A., JOLL, L.M. &
DREDGE, M.C.L. 1991. Australia. Develop-
ments in Aquaculture and Fisheries Science 21:
835-851.
HORTLE, M.E. & CROPP, D.A, 1987, Settlement of
the commercial scallop. Pecten fumatus (Reeve)
1855, on artificial collectors in eastern Tasmanian
waters, Aquaculture 66: 79-95,
TTO, H, 1988, Sowing culture of scallop in Japan. Pp.
63-68, In Sparks, А.К. (ed.), ‘New and innovative
advances in biology/engineering with potential
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NMFS 70),
JOLL, L.M. 1987. The Shark Bay SEM Fishery,
Fisheries Management Paper 11: 1-123,
JOLL, L.M. 1988. History, biology and managernent of
Westem Australian stocks of the saucer scallop
Amusium ballot, Pp,30-41. In M.C.L, Dredge,
W.F, Zacharin & L.M. Joll, (eds), ‘Proceedings of
the Australian Scallop Workshop, Hobart,
Australia’ (Tasmanian Government Printer:
Hobart).
KETTLE, В.Т, 1984, Settlement and growth of the Jocal
scallops Amusium pleuronectes (Linné) and
HATCHERY PRODUCTION OF WA SCALLOPS
Amusium balloti Habe. Hons, Thesis. James Cook
Uni. North Queensland. 93p.
MARU, K. 1985. Ecological studies on the seed produc-
tion of scallop, Patinopecten yessoensis. Journal
of the Hokkaido Institute of Fisheries 27: 1-53.
MASON, J. 1983. ‘Scallop and queen fisheries in the
British Isles’. (Fishing News: Farnham, Surrey).
MCDUFF, M.M. 1975. A study of some aspects of the
population ecology and reproductive biology of
Amusium pleuronectes (Linné) in coastal waters
of Townsville. Hons. Thesis James Cook Univer-
sity. 85p.
RHODES, E.W. & WIDMAN, J.C. 1980. Some aspects
of the controlled production of the bay scallop
(Argopecten irradians). Proceedings of the World
Mariculture Society 11: 235-246.
ROSE, R.A., CAMPBELL, G.R. & SANDERS, S.G.
1988. Larval development of the saucer scallop
Amusium balloti (Bernardi) (Mollusca: Pec-
tinidae). Australian Journal of Marine and Fresh-
water Research 39: 153-160.
ROSE, R.A. & DIX, T.G. 1984. Larval and juvenile
development of the doughboy scallop, Chlamys
(Chlamys) asperrimus (Lamarck) (Mollusca: Pec-
tinidae). Australian Journal of Marine and Fresh-
water Research 35: 315-23.
SAUSE, B.L., GWYTHER, D. & BURGESS, D. 1987.
Larval settlement, juvenile growth and the poten-
tial use of spatfall indices to predict recruitment of
275
the scallop Pecten alba Tate in Port Phillip Bay,
Victoria, Australia. Fisheries Research 6: 81-92.
SLACK-SMITH, S.M. 1990. The bivalves of Shark
Bay, Western Australia. Pp. 129-158. In P.F.
Berry, S.D. Bradshaw & B.R. Wilson, (eds), ‘Re-
search in Shark Bay. Report of the France-
Australe Bicentenary Expedition Committee.’
(Western Australian Museum: Perth).
SUMPTON, W.D., BROWN, I.W, & DREDGE,
M.C.L. 1990. Settlement of bivalve spat on artifi-
cial collectors in a subtropical embayment in
Queensland, Australia. Journal of Shellfish Re-
search 9: 227-231.
THOMPSON, D., BOURNE, N, & MANSON, C.
1985. ‘Scallop breeding studies’. (Pacific Biologi-
cal Research Station: Nanaimo, British Columbia,
Internal Report) 16p.
WELLS, F. & BRYCE, C.W. 1985. ‘Seashells of
Western Australia’. (Western Australian
Museum: Perth) 207p.
WILLIAMS, M.J. & DREDGE, M.C.L. 1981. Growth
of the saucer scallop, Amusium japonicum balloti
Habe in central eastern Queensland. Australian
Journal of Marine and Freshwater Research 32:
657-666.
YOUNG, P.C. & MARTIN, R.B. 1989. The scallop
fisheries of Australia and their management.
Aquatic Sciences 1: 615-638.
MODELLING MANAGEMENT MEASURES IN THE QUEENSLAND SCALLOP
FISHERY
M.C.L, DREDGE
Dredge, M.C.L. 1994 08 10: Modelling management in the Queensland scallop fishery.
Memoirs of the Queensland Museum 36(2): 277-282. Brisbane. ISSN 0079-8835,
The saucer scallop Amusium japonicum balloti is the basis of a trawl fishery with an average
annual meat production of about 1,000 tonnes in Queensland. A variable size limit (90mm
in summer and autumn, 95mm in winter and spring) and a ban on fishing during daylight are
significant components of the management package imposed on the fishery.
Effects of alternative management regimes on yield per recruit, value per recruit and
spawners per recruit have been evaluated using a modelling procedure. The effect of
variation in growth parameters has been interpreted in the model.
Results indicate that increasing the size limit to 95mm throughout the year would increase
spawners per recruit minimally while decreasing value per recruit 15-2096. A 95mm size
limit for most of the year with 24 hour a day fishing, would decrease spawners per recruit
by 15-35% in the ranges of F examined, while increasing relative value per recruit only at
lower exploitation levels.
M.C.L. Dredge, Queensland Department of Primary Industries, Southern Fisheries Centre,
P.O. Box 76, Deception Bay 4508, Queensland; 15 April, 1994,
Queensland’s saucer scallop (Amusium japon-
icum balloti) fishery is a component of the state's
multi-species trawl fishery. Some 900 trawlers
(10-20m long), are licensed to fish for a number
of species of penaeid prawns. slipper lobsters
(Scyllaridae), crabs and scallops. Catch and effort
data were monitored voluntarily in particular
fisheries, but have been compulsory and com-
prehensive since 1988. The current return system
calls for daily records of effort and catch within
30'x30' spatial grids, with some data being avail-
able at a finer spatial resolution (6' x6").
The trawl fishery annually takes c.10,000 ton-
nes of product, with variation in total production
and species proportions between years. Scallops
contribute an average of c. 1000 tonnes of meat
annually (Neil Trainor, pers. comm.).
All of the state’s licensed trawlers are legally
entitled to fish for:scallops. Not all vessels do
catch them, however. During 1988-1992, 270-
360 vessels reported catches of scallops (Neil
Trainor, pers, comm.), There appears to be excess
fishing capacity in the state's trawl fleet in the
context of taking the state's annual scallop catch.
Queensland's saucer scallop stock was first
fished in the mid 1950's (Rucllo,1975), when
prawn trawlers working out of Hervey Bay took
appreciable quantitics. The fishery remained an
irregular ‘off season' source of income for prawn
trawl operators unti] the mid 1970's, when
serious attempts to export saucer scallop meat to
the U.S. and south east Asia became profitable
(Dredge,1985). Effort directed towards scallop
stocks increased rapidly (Fig. 1), total catches in-
creased then levelled off, while catch rates
rrr da by an order of magnitude during 1978-
1985.
The scallop fishery has been managed through
both input and output controls, largely since 1985
(Table 1). Management was initially directed
towards maximising yield per recruit through the
use of a size limit, initially set at S0mm shell
height (SH), but later increased (Table 1). As
effort directed at the scallop resource increased
and catch rates fell, managers expressed concern
about the state of the resource and directed
management measures towards maintenance of
spawning stock levels. These included introduc-
tion of a variahle size limit (90mm SH in summer
and autumn, 95mm in winter and spring)
designed to reduce fishing effort during the
species” winter spawning season, introduction of
daylight trawl closures in order to reduce fishing
effort, and the short-lived trialing of areas closed
to fishing as spawning stock protection sites.
The industry has repeatedly expressed concern
about the management package which has
evolved over the past 10 years. Some fishermen
regard the daylight trawl ban us discriminatory,
Others would prefer to have a year-round size
limit of 90 or 95mm SH. An alternative proposal
involves 24 hour a day fishing while having a
278
MEMOIRS OF THE QUEENSLAND MUSEUM
1,800 Catch
1,600) Effort
Ж CPUE (ka/boat/day)
1,400
1,200 R
в 1,000 FN
c i f а
С {1 { ‚=
З воо i}
| à г X oet
і \ ^ 22" ү
an Ж-Ж | F + зй Y.
j \ + га
400, + A / "n эж.
* + ` 2 Y
200| / p ,"x-x*-* P
Lii:
Tue Ж-Ж у
of computer based
simulations. The results
= of this evaluation form
jV the basis of this paper.
MATERIALS AND
METHODS
Theoretical yield per
recruit outcomes from à
range of management
scenarios were model-
led using QuickBasic
programmes based on
those described in
5 ' Dredge (1992), The
programmes were struc-
tured to create a series of
overlaying two dimen-
sional matrices.
10
boat days (thousands)
AO A^ AB 49 © ох dr qo q* L d
95mm SH size limit for May-January, and a
90mm SH limit for February-April, inclusive.
These alternatives have been evaluated in terms
of meat yield per recruit, value per recruit, and
spawners per recruit, using outputs from a series
FIG.1. Total landings and catch rates from the nda saucer scallop fishery.
Tur MopEL
In the initial matrix,
one axis defined num-
bers in a series of
recruitment cohorts and the other defined time.
The resultant matrix developed a series of cohorts
linked to recruitment events over time. All
simulations were based on two identical, normal-
ly distributed recruitment pulses being fed into
TABLE 1. Summary of management "aL y in the scallop fishery (P. Pond, pers. comm.)
Gear size | ттам1 closures Designated Preservation zones
shucking areas
Daylight trawl
ban 1/10-31/1
Daylight Dy]
ban lifted
11/84 Combined
| һеайгоре апа
footrope <109т
784 | — 85mm č | [5m]
# BENE.
EIER
LN
EF FEET [|
90mm 11/90 - 4/91
Daylight trawl
each year Fue uni:
Гра сне —
& rawa n Ba
Three 10-minute ENS
10-minute areas
closed to fishing
ban
MODELLING MANAGEMENT OF QLD SCALLOP FISHERY
the model over a 16 week time period, in order to
simulate a four month, bi-modal spawning
process commencing in early winter
(Dredge,1981). The model was stepped through
the ‘time’ axis, both to feed in recruits and to
diminish the numbers of scallops in each cohort
through a process equivalent to natural mortality,
i.e. though the process № = Ni.e™, where Ni
represents numbers at time t, Ni+1 are numbers at
time t+1, and M the coefficient of natural mor-
tality. M was ascribed a value of 0.02 week"!
(Dredge, 1985),
The second matrix was used to estimate size at
age (shell height) in each cohort at each age.
Growth rates of scallops are known to vary with
location, apparently as a function of depth and
tidal regime (Williams & Dredge,1981; Dredge
& Robins-Troeger, unpubl. data). Three different
growth scenarios were used in this model. Sub-
sets of this matrix were used to ascribe size at age
for scallops from areas where growth was rapid
(Lc-1050 -g(0055*0y" intermediate (Li-100(1-e
(.051*. and slow (Li-97(1-e: ©"), with t in
weeks. Von Bertalanffy growth parameters were
derived from Williams & Dredge (1981) and
Dredge & Robins (unpubl. data). The model was
based upon 55% of scallop being taken from ‘fast
growth’ areas, 35% from ‘intermediate growth’
rate areas, and 10% from “slow growth’ areas.
These figures are based on the average spatial
distribution of catch for 1989-1991 inclusive
(Trainor pers. comm.).
A third matrix was used to convert shell height
to adductor weight for scallops in each cohort at
each age. Monthly shell heightto adductor weight
conversions, based on those in Williams &
Dredge (1981), were used for this procedure. A
dollar value was ascribed to scallops in each
cohort at each age by multiplying numbers of
survivors by meat weight by unit value of meat in
a fourth matrix. This required a correction factor
based on the individual meat weights, as there is
an appreciable difference in scallop prices based
on individual meat sizes (Hart, this memoir).
Fishing was simulated through a process which
involved identifying those cohorts in which scal-
lops were larger than a given (‘legal’) size and
increasing the mortality rate to include a com-
ponent for fishing mortality (F), ranging between
light (F=0.005 week!) to very heavy (F=0.040
week"). The resultant ‘catch’ of both meat
weight and value was accumulated as the model
was stepped through time.
An index of the number of spawning scallops
was developed by averaging the sum of the num-
279
value
Spawning index
value
spawn into
ees
zm жа 1000
+ = 2,800
an t га "ы-н! 1.
©
„| INTERMEDIATE Rey) -r
ù wo =
5 e
2 1500 Е
> E
100) 2
тот 5
л
E 2
ээ)
T маше
o C" узуп "i
^ >
& # g S $ rd 8?
Spawning index
ыу)
уче Ч
+ shaw ingen} oy
о А
n а ^ > 2 2 E
d o ef з Q9 у So =
Fishing mortality
FIG.2. Value per recruit and spawners per recruit from
scallops with differing growth parameters.
ber of one and two year old scallops which sur-
vived at the beginning and end of the winter
spawning period. This index has been used as an
index of spawners per recruit.
MANAGEMENT SCENARIOS
Yield per recruit in meat weight and dollar
value, and spawners per recruit were estimated in
the following management scenarios:
1) A 90mm SH size limit in summer and
autumn and 95mm SH size limit in winter and
spring, with no daylight fishing (the existing
management situation),
280
250- 35d
Size limit: 90 mm Nov ~ Арг, 85 mm May - Dot
300
=
200 3
За м
3 "—- 4250 o
р - ж + m
б = = es — 2 -
gon "LIBELLI Um
5 + a
a 10 t
ш 120 = >
= =
m ¥ 100 &
> a
50 in
T © value БО
E ® &
2
m D e
250 1200
Size limit: 85 men year round
p =A 250 =
_ Ж n aaa gs yn 3
5 B
8 wn ine
`~ | — gi
Ba | je 190 a
a Р -
mi # E
I + mz
[3
> - Я.
3 а ©
^V value
“"spaviner.
spa 0
Pod ^ ®
Dou wo gg
$
d SU g Ф = о?
No daylight closuré: assumg effective fishing aitort doubles
250 ую
ie limit: 90 mm Fab - Apr, 95 mm May - Feb Г
ШЕ
——_!— 250
- т
Е
1
1
a
"value 5n
Value per recruit
8
+
5
Spawners per recruit
а АЦА
* p e A d
u]
o e cv er er er ©”
бс
&
О.
Fishing mortality
FIG.3. Variation in value per recruit and spawners per
recruit as a consequence of varying management
regimes.
2) A year round 95mm SH size limit, with no
daylight fishing.
3) A 95mm SH size limit in May—January, and
a 90mm SH limit in February—April (inclusive),
with an increase in fishing mortality commen-
surate with 24 hour a day fishing throughout the
year. This was achieved by assuming that the
fishing mortality rate (F) doubled as a conse-
quence of allowing 24 hour a day fishing.
Model runs were carried out over a 104 week
time span, which approximates the effective max-
imum life span of the species (Heald & Caputi,
1981; Dredge,1985). Yield per recruit and
MEMOIRS OF THE QUEENSLAND MUSEUM
spawners per recruit were estimated as a function
of fishin g mortality (F) between 0.005-0.040
week .
RESULTS
Output from model runs is most readily inter-
preted in graphic form.
EFFECT OF VARIATION ON GROWTH PARAMETERS
Spawners per recruit and catch value per recruit
from equivalent recruitment processes were com-
pared for populations with 3 sets of growth
parameters. Variation in growth parameters had
an appreciable effect in ‘per recruit’ output (Fig.
2). Value per recruit of slow growing scallops
was 1/3-1/2 of that in fast growing scallops, with
the differential increasing as exploitation rates
increased. Conversely, spawners per recruit from
slow growing scallops remained at near steady
levels as the exploitation rate increased, indicat-
ing how few attained legal size before they com-
menced spawning. Spawner per recruit levels
from fast growing scallops declined from about
90% to 60% of those seen in slow growing scal-
lops as the rate of exploitation increased.
EFFECT OF VARIATION ON MANAGEMENT SCENARIOS
Variations in ‘per recruit’ output as a conse-
quence of varying management scenarios are
depicted in Fig. 3. If the output derived by
modelling the existing management situation is
used as a reference point, altering size limits from
90mm SH (summer and autumn), 95mm SH
(winter and spring) (management scenario 1) to
95mm SH all year round (management scenario
2) would result in a general decrease of 20-35%
value per recruit (rising with increasing F) and a
commensurate increase of 5-12 % in terms of
spawners per recruit. This is dependant upon the
relative input of fast, intermediate and slow grow-
ing scallops to the fishery.
Results from the model suggest that consequen-
ces of changing the management regime from 90
mm SH (summer and autumn), 95 mm SH (winter
and spring) (management scenario 1) to one in
which effective fishing mortality was doubled
(no daylight closure) and size limits were held at
90mm SH in Febniary to May, and 95mm SH for
the remainder of the year (scenario 2), value per
recruit would increase substantially ( 40 %) at low
levels of exploitation, but change little at higher
exploitation levels. Spawners per recruit would
be reduced by about 10% at lower levels of ex-
MODELLING MANAGEMENT OF QLD SCALLOP FISHERY
ploitation, and up to 25% at the highest exploita-
tion level examined,
DISCUSSION
The model demonstrates that yield per recruit
and spawners per recruit will be markedly in-
fluenced by growth parameters of scallops taken
in the fishery. There is evidence that growth
parameters of scallops may vary considerably
over relatively small distances (Williams &
Dredge,1981; Ansell et al..1991; Ciocco, 1991).
Such variation in growth parameters can have a
marked effect on optimum age or size at first
capture for yield per recruit maximisation. Varia-
tion in growth parameters have been recognised
and incorporated into the model described in this
paper. There is, however, no reason why propor-
tions of landings from slow, intermediate and [ast
growing areas should remain constant, and con-
sequently the model's output should be treated as
indicative. In years when a high proportion of
landings come from areas where scallops grow
quickly, vield per recruit may be maximised by
having a larger size limit, and conversely, when
scallop settlement occurs predominantly in ‘slow
growth’ areas, yield would be increased with a
smaller size limit. Given the lead-in time and
information requirements for a management sys-
tem using flexible size limits, implementation
seems unlikely in the short term.
The output derived by modelling the fishery
under alternative management scenarios indi-
cated that variation in exploitation levels effected
both trends and absolute values of yield per
recruit.
Dredge (1992) suggested that a size limit of
Ymm SH maintained throughout the year would
have litile effect on value per recruit by com-
parison with the existing 90mm SH (winter and
spring), 95mm SH (summer and autumn) size
limits. Output from the model used in this study
suggested that increasing size limits to 95mm SH
on à year round basis would induce a substantial
loss to the fishery with a relatively minor increase
in terms of spawners per recruit,
The management option involving size limits
being set at 90mm SH in February to May, and
95mm SH for the remainder of the year, and
increasing exploitation by allowing 24 hour a day
trawling was examined, Results indicated that
spawners per recruit would be reduced by 10-
25%, and valuc per recruit would be increased
only at bow levels of exploitation. This scenario
involved a fair degree of uncertainty, however, их
281
an arbitrary doubling of fishing mortality was
used to simulate the effects of allowing 24 hour а
day tràwling. Verification of such an arbitrary
procedure is not possible.
Given the current limitations in our under-
standing of spawning stock and subsequent
recruitment levels, the model output indicates
that the existing management package offers a
reasonable compromise between obtaining max-
imum catch value from the resource while main-
taining brood stock levels.
By comparison, the Western Australian agency
which manages a fishery for the same species has
amanagement philosophy based on limited entry,
minimising capture costs, and minimising con-
flict between alternative fisheries in the main
fishing ground (Shark Bay). Maintenance of a
substantial breeding population is considered
critical to management. This is achieved by
having a summer closure, and a predominantly
winter fishery, hus allowing the bulk of animals
to spawn early in the (winter) spawning season.
(Joll,1987,1989}, Size limits are not used in the
WA fishery, as scallop shucking in the fishery
takes place at sea-
Queensland fisheries managers seek to main-
tain biological sustainability and long term
economic viability of the (integrated east coast
trawl) fishery while recognising social values in
defining management actions (Glaister et al.,
1993), The differences in management phil-
osophy have resulted in fisheries which have
markedly different seasonality, input costs and
numbers of participants.
ACKNOWLEDGEMENTS
Neil Trainor provided many of the cateh and
effort statistics from the Queensland commercial
fisheries data base, SUNFISH. Julie Robins-
Troeger prepared the graphics, and the editorial
committee at Southern Fisheries Centre reviewed
the manuscript, My thanks Io all of these people
for their efforts.
LITERATURE CITED
ANSELL, A.D,, DAO, J.C. & MASON, J. 1991. Three
scallop fisheries; Pecten maximus, Chlamys (Ae-
quipecten) opercularis and C. (Chlamys) varia. ln
S.E, Shumway (ed.), 'Scallops: biology, ecology
and aquaculture’. (Elsevier; Amsterdam).
CIOCCO, N.F. 1991, Difference in individual growth
rate among scallop (Chlaemnys tehuelcha (d'Orb])
populations from San Jose Gulf (Argentina).
Fisheries Research 12; 31—42.
282
DREDGE, M.C.L. 1981. Reproductive biology of the
saucer scallop Amusium japonicum balloti (Ber-
nardi) in central Queensland waters. Australian
Joumal of Marine and Freshwater Research 32:
775-787.
DREDGE, M.C.L. 1985. Estimates of natural mortality
and yield per recruit for Amusium japonicum bal-
loti Bernardi (Pectinidae) based on tag recoveries.
Journal of Shellfish Research 5(2): 103-109.
DREDGE, M.C.L. 1992. Using size limits to maintain
scallop stocks in Queensland. In D.A. Hancock
(ed.), ‘Legal sizes and their use in fisheries
management'. Australian Society for Fish Biol-
ogy Workshop, Lome, 24 August 1990. Bureau
of Rural Resources Proceedings No. 13.
(Australian Government Publishing Service, Can-
berra).
GLAISTER, J.G., POND, P.C & STOREY, J.G. 1993.
‘Framework for management for the East Coast
trawl fishery’. (Queensland Fish Management
Authority: Brisbane).
HART, B. this memoir. Dilemma of the boutique
Queensland scallop.
HEALD, D.I. & CAPUTI, М. 1981. Some aspects of
MEMOIRS OF THE QUEENSLAND MUSEUM
growth, recruitment and reproduction in the
southern saucer scallop, Amusium balloti (Bernar-
di,1861) in Shark Bay, Western Australia.
Fisheries Research Bulletin of Western Australia
25: 1-33.
JOLL, L.M. 1987. The Shark Bay scallop fishery.
Fisheries Department, Western Australia,
Fisheries management Paper 11.
JOLL, L.M. 1989, History, biology and management of
Western Australian stocks of the saucer scallop
Amusium balloti. In M.C.L. Dredge, W.F.
Zacharin & L.M. Joll (eds), ‘Proceedings of the
Australasian scallop workshop'. (Tasmanian
Government Printer: Hobart).
RUELLO, N.V. 1975. An annotated bibliography of
prawns and the prawning industry in Australia. In
P.C. Young (ed.), ‘First Australian prawn
seminar’, (Australian Government Publishing
Service: Canberra).
WILLIAMS, M.J. & DREDGE, M.C.L. 1981. Growth
of the saucer scallop Amusium japonicum balloti
Bernardi in central eastern Queensland.
Australian Journal of Marine and Freshwater Re-
search 32: 657—666.
SCALLOP FISHERIES, CULTURE AND ENHANCEMENT IN THE UNITED STATES
SANDRA E. SHUMWAY AND MICHAEL CASTAGNA
Shumway, S.E. & Castagna, M. 1994 08 10: Scallop fisheries, culture and enhancement in
Ше опна States. Memoirs of the Queensland Museum 36(2): 283-298. Brisbane. ISSN
-R835.
Information is provided on distribution, commercial landings and landed value of: sea
scallop, Placopecten magellanicus, bay scallop, Argopecten irradians, calico scallop, Ar-
gopecten gibbus, om scallop Chlamys rubida, spiny scallop, Chlamys hastata and weather-
vane scallop, Patinopecten caurinus, Where applicable, information is provided on fishing
regulations and management plans, Aquaculture of scallop is limited to a few ventures
utilizing the bay scallop, A. irradians. Enhancement programs are designed to reinstate
populations of A. irradians to areas decimated by the "brown tide' Aureococcus anophagef-
Jerens and regional efforts to provide some stability to local fishing efforts,
Sandra E. Shumway, Bigelow Laboratory for Ocean Sciences, Wesi Boothbay Harbor,
Maine 04575, USA (present address: Natural Science Division, Southampron College LIU,
Southampion, New York 11968, USA) & Michael Castagna, Virginia Institute of Marine
Science, Wachapreague, Virginia 23480, USA; 20 June 1994,
Scallops are commercially important shellfish
worldwide (Table 1); US landings of all scallops
were 40 million pounds of meat (18,000tonnes)
valued at $US162.5 x 106 for 1991 (O'Bannon,
19913). This represented a decrease of 1.6 million
ounds (700 tonnes) (4%) but an increase of
US4.4x105 (3%) compared with 1990. Four
species (sea scallop, calico scallop, bay scallop,
and weathervane scallop) contribute to the major
wild fisheries in the US with minor fisheries for
pink scallop and spiny scallop. In 1983 and 1987
Massachusetts reported 418,800 and 29,400
pounds (190 and 13tonnes) respectively) annual
landings of the Icelandic scallop, Chlamys islan-
dica and Rhode Island reported landing 2,800
pounds (1.2tonnes) of this species in 1983. C.
islandica is not regularly fished in US waters.
Aquaculture and enhancement efforts are
limited activities in the US to the extent that
scallop aquaculture is not even listed by FAO in
their annual statistics reports (FAO, 1992); how-
ever, where these activities do occur they con-
tribute to local economies. Further, production
from domestic activities (fisheries, aquaculture
and enhancement) does not totally meet supply
requirements and scallops are regularly imported
from other countries (Tables 2,3).
We present a brief overview of US scallop
fisheries, aquaculture and enhancement efforts, 1
is not intended to be comprehensive,
COMMERCIAL FISHERIES
Sea $сА ОР, PLACOPECTEN MAGELLANICUS
This large, long-lived species attains shell
heights of 8.5ins (20cm) and supports an inten-
sive fishery throughout its range from New-
foundland to North Carolina. American cam-
mercial fishing efforts centre on Georges Bank,
coastal New England and mid-Atlantic states
(Naidu,1990; Fig.1). The fishery is 2100 years
old and P. magellanicus is the most important
pectinid in the world (Naidu,1990). During
1976—1987 it accounted for 30% of mean annual
global production of all scallop species combined
(Table 1). In some years, P. magellanicus has
contributed >0.5 of global scallop production.
Enhancement of some species (particularly the
Japanese scallop, Patinopecten yessoensis) and
sporadic booms in natural production of calico
scallops (A. gibbus} have relegated sea scallop
landings to a seemingly secondary role. The ad-
ductor muscle (meat) is the only portion com-
monly marketed in the US. although there is
steady interest in developing a “roe-on' product.
Sea scallops comprise the bulk of scallops
landed in the US (Table 2) with New Bedford,
Massachusetts being the leading producer in
1991, landing of 21.9 million pounds (10,000
tonnes) of meats (56% of national total) (O'-
Bannon, 19922). The average ex-vessel price per
pound of meat increased from $1053.85 ($051.75
fkg) in 1990 to 5054.04 (SUSI.84/kg) in 1991,
Total catch and landed values are given (Figs 2,3;
Table 4). Regional landings vary; the New
England region consistently produces most scal-
lops and more southerly regions the least (Fig.2).
The commercial fishery operates year-round
using otter trawls and dredges. Recreational
284
MEMOIRS OF THE QUEENSLAND MUSEUM
TABLE 1. Nominal landings (MT, round weight) of scallop species. Figures in parentheses are % contribution
to global production in any given year. Source: Yearbook of Fishery Statistics, FAO, Rome, Voi. 70.
| Chlamys islandica 17.068
(Iceland scallop) 1.8) (2.8)
Chlamys opercularis
queen scallop)
(1.6 (LR
238,236
25.6
Patinopecten yessoensis
Japanese scallo 39.4
3,649
DILLON NE |
| (weathervane scallop) (0,6) (0.6)
Pecten maximus 22,253 20,128
pliant scallop 2.6
3.3)
Pecten jacobeus 7
(Pilgrim's scallop) =
*species fished commercially in the US.
fisheries are rare and occur predominantly in
Maine where scallops are collected by divers.
Management of sca scallop resources has his-
torically been a local issue. US scallop manage-
ment efforts started When Maine imposed a
summer closure sometime between 190] and
1917, Many local management regulations are
still in effect and many more have been imple-
mented to conserve stocks and control gear con-
flicts. No regulation of the offshore fishery
existed prior to 1983 other than what the industry
imposed upon itself. In inshore waters, scallop
management has existed for a long time (Shum-
way & Schick,1987). Maine has had long-stand-
ing regulations for conservation of scallop stocks
within its 3mile territorial limits; New Hampshire
has had conservation regulations of a 3.251nch
(8.25cm) minimum shell height and an April 15
through October 31 closed season since 1977.
Massachusetts, with the largest offshore scallop
fishery out of New Bedford, has had no regula-
tions as it has no large inshore beds nf sea scal-
[specs — | i94 | 1985 | ios | 1087 | 1988 | 1989 | 1990 |
| (Atlantic calico scallop) 47.2) (20.8) (3.2 (11.6 (14.0) 8.0) (1.3)
* Argopecten irradians 6,597 2904 І y
bay scallop 0.8) (0.8) (0.9 (0.4) А
Chilean scallop) (2.8) (8.5) (3.1) (0.8 (0.9) (0.5 (0.9)
16,429 | 13,385 12,117
(3.1) ((1.8) (1.2) (L3) 14)
22) 1.9) 1.8) (6 (2.0)
(0.5) (0.4) (0.1 (0.2) (0.3)
17,353 15,357 15,812
3 (2.1) L8 (19)
ир
New Zealand scallop (0.6 0.5 33 0.1 0.1 0.1 9.1
sea scallop) 12.3 17.4) (24.5) 26.2 22.3) (24.5) 124.7)
| 4.0 39) | (62) | (81) 38 | пә) | (23
[World rota | 838,067 | 604215 | 530.739 | 738,606 | 868,005 | $4023 | 876636 |
7,467
2,329 2,596
(0.3 0.2 03
502,136
59.8)
571,003
65.1
lops. Each state has modified their regulations to
at least comply with the US federal regulations
for the Fisheries Conservation Zone (FCZ), but
Maine's regulations remain even more restric-
tive, with specific area restrictions or season,
geartype and gear size, a ban on nighttime shing
for scallops, drag size limits which vary with
season and a requirement for a hand-fishing
license for divers and a boat license for draggers.
Regulations in the US offshore scallop fishery,
which includes Georges Bank, Gulf of Maine
and mid-Atlantic Bight as far south as Cape
Hatteras (Fig.1) have been imposed by industry
in thc form of crew size, maximum allowable
time at sea per trip, minimum time at the dock
between trips and a maximum of two tows
dumped on deck at one time prior to shucking.
With advent of the 200 mile Fisheries Conserva-
tion Zone, New England and mid-Atlantic
Fisheries Management Councils developed and
implemented the Sea Scallop Fisheries Manage-
ment Plan (FMP) to regulate the fishery, The
US SCALLOP CULTURE AND ENHANCEMENT 28
cn
TABLE 2. U.S. supply of scallop meats 1972—89) (meat weight in million pounds) (after Dore, 1991)
mI S. commercial IUE gs Total Imports "Total Percent
| Calico | =a Supply Imports
basis for managing the Georges Bank, Gulf of
Maine and mid-Atlantic scallop fisheries under
the FMP has been to increase yield per recruit by
controlling age/size of recruitment by imposing a
maximum average meat count. The FMP was
implemented in May,1983, and imposed a 30
meat count per pound maximum with an
equivalent shell height of 3.51nches (8.9cm). The
TABLE 3. Domestic and imported scallop species on
the U.S. market
DOMESTIC
Placopecten magellanicus
Calico scallop Argopecten gibbus
| Bay scallop Argopecten irradians
[Bay scallop — |
[Weathervane scallop |Parinopecten caurinus |
[Spiny scallop (Chlamys hastata |
IMPORTED
[Bay scallop [Argopecten irradians |
[Peruvian scallop [Argopecten purpurams |
Sea scallop
——— —
Regional Director of National Marine Fisheries
Service (NMFS) immediately increased the count
to 35/pound with a shell height minimum of
3.275inches (8.6cm) due to the unwillingness of
Canada to go along with a 30 count maximum.
This temporary change in the limits was to be in
effect until January, 1984, when the limits would
go to 30 meats per pound and 3.5inch (8.9cm)
shell height. The 30 count regulation was delayed
until January 1986 due to industry and political
pressures and the 35 meat count was retained.
Under this scheme of an average meat count,
small scallop meats may be mixed with large
meats as long as the average meets the maximum
count requirement.
In 1984, a large set of scallops in the Great
South Channel of Georges Bank promised to
sustain the scallop fishery for some time; how-
ever, most of this set was harvested at a small size
and the meats were mixed with larger meats to
achieve the 35 count maximum. Almost the entire
set was harvested at well below its potential yield
per recruit and before it was able to significantly
contribute to reproduction. To prevent this from
happening again, the Councils proposed Amend-
ment | to the FMP that would institute a 40 count
minimum meat size, which would create an
286
75° Ww 70° 65
o
MEMOIRS OF THE QUEENSLAND MUSEUM
ES
27 S,
т
kd
А |
g NEWFOUNDLAND
50°N
GULF OF
ST LAWRENCE
40*
DISTRIBUTION, COMMERCIAL QUANTITIES
OCCUR OCCASIONALLY
COMMERCIAL FISHING, FREQUENTLY
PERSISTENT
o
60
o
55
FIG.1. Distribution of the sea scallop, Placopecten magellanicus, and commercial fishing grounds.
average meat count of around 30, but would
prohibit the mixing of scallops much smaller than
the minimum size. This effort brought much
criticism from the industry.
Amendment 1 to the FMP went into effect on
January 1,1986, but was delayed by the Regional
Director of NMFS and was rescinded May
28,1986. Scallop management then returned to
the FMP and the 30 count average with a 10%
tolerance (effectively a 33 count average) and
3.5inch (8.9cm) shell height was imposed. The
shell height of 3.5inches is based on an average
shell height to meat weight regression showing
the shell height for a 30 count scallop meat.
Industry criticism has been levied against the
3.5inch (8.9cm) shell size as well. The industry
arguments centred on the fact that the shell height
to meat weight relationship is highly variable
from location to location and from season to
season (Fig.12; Ѕегсһик,1983; Serchuk & Rak,
1983; Schick et al.,1988). With scallop sets occur-
ring at different locations in different years, or
even in the same year, having one shell height to
meat weight regression represent the whole
fishery they claim is unreasonable. Currently
shellstockers can harvest scallops in the mid-At-
lantic Bight at 3.5inch (8.9cm) shell height that
have meats too small for the at-sea shuckers to
US SCALLOP CULTURE AND ENHANCEMENT
40
30
=”
2°
m
20
->
|
|
| "
|
|
|
—.
TOTAL LANDINGS ( POUNDS x 108 )
IT
' ' T
50 54 58 62 66 70 74 78 82 86 90 94
YEAR
FIG.2. Landings of sea scallop, Placopecten magel-
lanicus. Data from O'Bannon 1992b)
harvest even at 33 count. With the large recruit-
ment of recent year classes producing a bonanza
for the shellstockers and little for the at-sea
shuckers, there is much asperity in the industry
with cries of unfair management practices.
In response to industry criticism, the Councils
put forth Amendment 2, which contains options
for management of the scallop resource. During
several hearings industry spokesmen made it
clear that most options were untenable, or at least
unpalatable to them. Current regulations require
à 30 average meat count per pound standard for
shucked scallops and a3.5inch (8.9cm) minimum
shell height standard for unshucked scallops.
‘Fishing effort on Georges Bank is at record
levels and far beyond what the resource can sus-
tain in the long run’ (Anonymous, 1992).
Discussions are now focussed on implementa-
tion of Amendment #4 (designed to replace the
meat count system) which includes the following
common elements (Commercial Fisheries News,
Dec. 1992): a moratorium restricting entry into
the fishery; maximum crew size of nine, includ-
ing the captain; 3.25inch (8.3cm) ring size mini-
mum that would increase to 3.5inches (8.9cm) the
third year of the plan; 5.5inch (14,0cm) minimum
mesh size for trawl gear; 30 foot (9.2m) limit on
160
Sea Scallop
120
eo
e
TOTAL VALUE ($ US x 10°)
50 54 58 62 66 70 74 78 82 86 90 94
YEAR
FIG.3. Landed value of sea scallops, Placopecten
magellanicus (meats). Data from O'Bannon 1992b)
the total width of all dredges and a 144 foot (44m)
limit on the sweep of trawl gear; no onboard
shucking and sorting machines on boats that land
shucked scallops; continuation of the 12h landing
windows and no at-sea transfer of scallops; con-
tinuation of the 3.5inch (8.9cm) minimum shell
height standard for shellstockers (fishermen who
land scallops in their shells); no chafing gear,
cookies orother devices which obstruct the top or
sides of the scallop dredge and a 5.5inch (14.0cm)
minimum twine top on top of all dredge gear;
annual permits and mandatory data reporting for
vessel owners, dealers, brokers and processors as
weli as licenses for vessel captains; continuation
of the meat count as an alternative to the follow-
ing gear restrictions: increased ring size, 5.5inch
(14.0cm) trawl mesh, 5.5inch (14.0cm) twine top,
and prohibitions on chaffing gear, cookies and
other obstructing devices. In additions, there arc
four alternatives proposed: 1) (preferred) limited
days at sea by vessel group (full-time fleet, part-
time fleet, occasional fleet); 2) limits on days at
sea; 3) adjustable trip limit with fixed layover; 4)
fixed trip limit with adjustable layover.
Inasmuch as the goals of management are to
optimize yield while at the same time stabilizing
288 MEMOIRS OF THE QUEENSLAND MUSEUM
TABLE 4. Historical catch statistics (total catch by regions) for sea scallops, (Placopecten magellanicus), for the
period 1950—1991 (numbers in thousands). (O° Bannon,!992b)
| Year | NewEngland | Middle Atlantic | ^ Chesapeake | “South Atlantic | _ To |
Rounds | Dci | Pomas [Dele | Tode | Dol. | Pounds | Dotty | Tess | реа |
| 1950 | 13753 | 6384 | 6135 | 2781 | 92 | 39 | — | — | 19980 | 9204 |
osi | маз | ееп | aoso | з | єз | ж | с алав вза
| 1952 | 15392 | 9093 | 3205 | 1721 | 32 | m | — | — | 18629 | 10,832 |
Les p sse aee [ase [ims | «4 БИП | — оза | золе |
| 1954 | 15.594 | 7.028 | 207 | 9: | — | — | — | — | wes 7976 |
| 1955 | 16848 | 8821 | 5244 | 2610 | 33 | wm | 2 | 2 | 22125 | 1449 |
| 1956 | 16,881 | 9109 | 3164 | 1700 | 21. | з | — | — | 200% | 10,822
| 1957 | 18,781] 9119 | 2167 | 1040 | в | 21 | — | — | 20994 | 10.80 |
| 1509 | 16410 | 7941 | 2324 | 1097 | 243 | 102 | — | — | 18977 | 9140 |
| 1959 | 20259 | 9.05 | 3949 | rgia | 436 | в | — | — | 24,684 | 11,805 |
[ 1960 | 22462 | 7,823 | 3356 | 1153 | 781 | 290 | — | — | 26599] 9266 |
| 196i | 23.775 | 9,035 | 3.368 | 1.238 | sis | п | — | — | 2746) | 10404 |
Lm» | зын | sasz | 288 | LM | os | M | авы | ioon
[195 | rom | 8257 | 2099 | sm | 4e | | — | — | 19939
WEE я: ииз ENT NNNM MORE иии
с 9 —— аана. ыз | ам ыш уыш onm ы | в | m, Due
[1965 | uir | 5.520 | 2528 | 1186 | 2300 | | — | — | 15975 | 265 |
RC DES GET ДЕТТИ ЕТТИН AENEA и
| 1968 | 7938 | 8850 | 1,978 | 2194 | 2112 | 2268 | 42 | 42 | 12070 | 13354 |
A з= P — es
Lm | aser | ems] es | us | w | ws | — | — [3з | ras]
[1971 | 4346 | sais | sa | m [| sas | sor | — | | 5406 | 7991 |
| 1972 | 4422 | 8628 | ass | 933 | 960 | 1856 | — | — | 5850 | 11.417 |
| 1973 | 3949 | 7072 | seo | 1067 | m | 3347 | — | | 5291 | 9486 |
| 1974 | ави | 7174 | s34 | во | #72 | 127% | — | — | 6017 | 9267 |
| 1975 | 7,081 | 1382 | 981 | 1780 | 1270 | 2330 | 421 | 421 | 9753 | 17,913 |
3627 | 5529 | 67 | 954 | 25831 | 41,951_|
| 1979 | 16202 | 55,037 | 5772 | 18,717 | 7676 | 24376 | 1,694 | 4989 | 31,344 | 103,028 |
| 1980 | 17.018 | 65.571 | 4.143 | 16274 | 6140 | 23.776 | sei | 2,979 | 23.162 | 108,600 |
| 1981 | 19.910 | 30212 | 2,570 | 10709 | 3,350 | 14467 | 125 | am | 25,055 | 105,866 |
| 1982 | 15,822 | 58,995 | 1,920 | 7244 | 2.194 | 8370 | 2 | 1 | 19,936 | 74,590 |
| 1983 | 13,574 | 76385 | 2.719 | 15436 | 2515 | 16206 | 26 | 151 | 19.234 | 108,268 |
| 1984 | i124 | 62652 | 2,573 | 13803 | 394 | 17747 | 170 | вв | 17,191 | 95,08 |
50,078 | 1,849 13380
| 1986 | 11,707 | 61669 | 2317 | 10388 | 4264
3,558 men
30301 | 128495 |
KCNETTNETSNTONET HE: заа ие. 4.036 | 39.275 | 151.67
[ 9 | 25081 | 101932 | asas | mie | вазї | 32.807 | вз | 2324 | 38362 | 155272 |
u
ia
US SCALLOP CULTURE AND ENHANCEMENT
KS Argopecten irradians
ў amplicostatus
289
Placopecten magellanicus []
Argopecten irradians
irradians
; ; NI
Argopecten irradians
concentricus
3 Argopecten gibbus
FIG,4. Distribution of bay scallops, Argopecten irradians.
the catches, it seems reasonable that considerable
attention should be paid to the high level of
variability that can occur in meat weight within a
given fishing area. Since a single meat count is
not going to be valid ‘across the board’, different
meat count and/or sheli height regulations are
needed for separate fishing zones. It is further
suggested that, since seasonal and yearly varia-
tion in meat weights have been demonstrated,
meat count regulations should be based on yearly
sampling and set on a seasonal and area-specific
basis. While a constantly changing count/size
limit will cause problems with regard to com-
pliance and enforcement, it will strip away in-
equities between harvesting techniques and in-
crease yield to the fishermen by effectively in-
creasing yield-per-recruit and allowing
management closer to the limits of the resource.
Atatime when the scallop fishery is increasing,
and for a species which experiences such drastic
fluctuations, management cannot be too careful
in the regulations it imposes.
Bay SCALLOP, ARGOPECTEN IRRADIANS
The species range is discontinuous along the
Atlantic coast of North America between Nova
Scotia and Colombia. A. irradians irradians oc-
curs from Cape Cod to New Jersey where it is
790
3.5
Bay Scallop
3.0
1
і
254 Íl
ae К
4 А |
8 20 i3 [ "n
с Ji $
g 157 FN
5 Е: ‘il i
z "gd a E
5 i ‘9 y. |
gw! NT .I
5 on "
» 4
0.5 i „i
A +
\ 4 Pape ANY ед
a ix м, А, Ner
50 54 58 62 66 70 74 78 Be BG 90 94
YEAR
0
FIG.5. Landings of bay scallops, Argopecten ir-
radians. Data from O' Bannon 1992b).
replaced by A. irradians concentricus which ex-
tends from New Jersey to Florida. A. irradians
amplicostatus is found in the western Gulf of
Mexico to Colombia (Fig.4). While this species
represents only a minor component of US com-
mercial fisheries (Tables 1,2), it is extremely
important to local economies.
Rhodes (1990) reviewed the biology and
fishery of A. irradians which is a small, short-
lived species, usually spawning only once; how-
ever, a second spawning by some individuals
takes place in some regions. They occur in shal-
low water («10m) in protected bays and estuaries,
reaching a size of c,4inches (10cm) in 16 months.
Meat counts are 50-100/pound (23—45/kg).
Landings vary between seasons (Table 5) and
populations are dependent upon natural recruit-
ment for continuation, although some enhance-
ment efforts have been attempted. In 1985, bay
scallop populations in the northeast were
decimated by blooms of a previously unknown
microalga, Aureococcus anophagefferens
(brown tide’) (Tettelbach & Wenczel,1993;
Fig.11). Three successive years of algal blooms
resulted in virtually all native stock in the Peconic
Bays and the New York fishery being eliminated,
Eelgrass beds were also depleted, reducing the
MEMOIRS OF THE QUEENSLAND MUSEUM
TOTAL VALUE (5 US x 10°)
0 Sim - - - ў A: ^.
50 54 58 62 66 70 74 78 82 86 90 84
YEAR
FIG.6, Landed value of bay scallops, Argopecten
irradians, Data from O'Bannon 1992b)
total area of suitable habitat for scallop settle-
ment. Landings for 1991 were 438,000pounds of
meats (200tonnes) valued at $US2.7 million. This
isa decrease of 101,000 pounds (46tonnes) (19%)
and $US436,000 (14%) compared with 1990 (O'-
Bannon,1992a). Massachusetts was the leading
state with 375,000 pounds (170tonnes) of meats,
86% of the national total. The average ex-vessel
price was $US6.09/pound ($US2.77/kg) of meats
compared with $US5.76 ($US2.62/kg) in 1990
(Figs 5,6: Table 5).
Commercial fishing records for A. irradians
date back to 1858 (Ingersoll, 1886) and the intro-
duction of the dredge іп 1874. Commercial fishing
for A. irradians is strictly limited and there is a
large recreational fishery. Harvest is usually
limited to September-December. In most areas,
the bay scallop fishery is a protected resource.
Scallops are usually collected by diver, hand-
picking or rake. Some fishermen use small boats
equipped with outboard engines and one or two
small dredges. Scallops are culled on board and
only the meats are harvested. Catch limits are
determined on a season-by-season basis hy
fisheries officials in accordance with population
fluctuations (Rhodes, 1990).
US SCALLOP CULTURE AND ENHANCEMENT 291
TABLE 5. Historical catch statistics (total catch by regions) for bay scallops (Argopecten sp.) for the period
1950-1991 (numbers in thousands). (O'Bannon,1992b)
|| Pounds | Dollars | Pounds | Dollars | Pounds | Dollars, | Pounds | Dollars | Pounds | Dollars |
NM e
| 1951 | 1253 | 959 | 101 [| 123 | 183 | % | 252 | 161 | 1789 | 1337 |
| 1952 | iiss | 913 | 182 | 255 | 254 | 126 | 20 | as | 1834 | 1342 |
| 1953 | 2397 | 1222 | 162 | 102 | вз | 33 | 29 | s3 | 2853 | 1410 |
| 1954 | 987 | бз | 127 | no | s [| в | a | 10 | 120 | 83 |
| mss | 1070 | s37 | 2% | no | w | 39 | 2з | 53 | 1,97 | 1139 |
[ mss | з | am | aos | 4% | vs | 6s | m | w | 1300 | 9o |
Съз [ыз [зш | вы [жт [| | ar | as CONSE |а |
Cum [ims] we | sm | n: [19 | x | 4 | m [om [з |
Ls | si | mo | os | ж | ps | у | 8 | э | ы ые
| 1960 | 1065 | 759 | мз | вм | ө | 27 | se | 14 | 2031 | 1474 |
| w | en | se | вл | me | a | 36 | за | 198 | 1348 |
| 1962 | 1425 | ові | 1353 | ssi | 18 | c: | эз | 68 | 3159 | 2067 |
| mes | зәт | 492 | sm | ам | зи | 122 | 28 | s | nsu | 107 |
—9 | в | sos | iw: | se | wo | TG | в | ы foar | neee
| 1955 | aso | sez | om | 766 | 379 | 1% | so | за [| nes | 158 |
2s | mo | ume | e: | в | we | m |. 9 | + | мш | en |
[1m | 4s | sm | ms | ow | a | zu | 7 | s | 1,007 | 1053
N CHE NE NE UM | zs | 34 | e | 4m | 1з | 122 | 1491 | 1694 |
зв [um [im ш | m | ex | ss | m | e Гам оез |
om |o f aos | зе | шю | о | э | юш ж | мю мт
[ 197: | 2063 | 3531 | 144 | 24 | co | 42 | зв | 39 | 2315 | 386]
| 1972 | i776 | 3407 | 9» | zi | i | no | 35s | м | 2032 | 3,772 |
[195 | æa | 14e | zo | 467 | x | s | s | e | 104 | 2005 |
[ 194 | sez | һом | c | sm | 20 | 199 | в | w | 1497 | 2103 |
iere age РЕТ аа ЭИУ Toc ie aa
1926 | we [im e | me | om | 1 | a | a | 25 | 39 |
[ 197 | пола | 3.085 | 199 | ago | 257 | so | в | s | ns | am |
| 1978 | 152) | 4982 | 2:0 | sx | 221 | зз | 49 | o | 207
1979 | 1.382 | s967 | sas | i20 | їз | sia | 6 | зт | 1983 | тве |
[1999 | 1356 | ввп | ам | аво | эв | igor | п | 29 | 2126 | 9607 |
ИШЕ RE IE BA BS ин BE BEBE ии S
| 1982 | 2022 | 8949 | soo | 1809 | WI шш ши ши жин Гава RET
| 1983 | 1083 | &a9i | 167 | 99 | | эж | z | 75 [| m | 8067 |
E IB HECUHROBT Ne HN CBE NE HET ОЕ E
[ 1985 | sss | sai2 | 174 | sm | ase | 1072 | a | 10 | 1,592
[в | so | ay | 13 | в | so | вз | от | se | sss | 4786 |
Lem | эш zm] 2: | э еа | в | ю | зз | эз
608 3, a8]
EE E 215
Zr
292
Gulf of Mexico
Key West
85°
MEMOIRS OF THE QUEENSLAND MUSEUM
{ Cape
^. Hatteras 7| 35°
Lookout
Charleston
А 30°
St. Augustine Atlantic Ocean
Cape
Canaveral
25°
80° 75°
FIG.7. Commercial fishing grounds for the calico scallop, Argopecten gibbus (after Blake & Moyer, 1990)
CALICO SCALLOP, ARGOPECTEN GIBBUS
This species supports a variable fishery off
Florida (Fig.7). Locations of commercial stocks
vary from year to year; however, Cape Lookout,
Cape Canaveral and Cape Sand Blas are key
areas. The fishery and biology were reviewed by
Blake & Moyer (1990). The scallops grow to
<3inches (7.5cm) and the adductor muscle (meat)
is small and brownish (meat count 100—300 per
pound; normally 150—200). Hand-shucking is not
economically feasible; thus, even though large
stocks of calico scallops were known as early as
1949, the species was not harvested commercial-
ly prior to automation in the late 1970's.
During its peak (1984), landings exceeded 39
million pounds (17,700tonnes) and the fishery
was almost non-existent in the late 1980's and
early 1990's (Tables 1,2; Fig.8). Annual varia-
tions in production impact not only the total US
catch, they also determine the position of the US
among world scallop producers. Landings were
286,000 pounds (122tonnes) of meats valued at
$US858,000 in 1991. According to O'Bannon
(19922), this represented a decrease of 849,000
pounds (390tonnes) (7576) and $US423,000
(33%) compared with 1990. All calico scallops
were landed on the east coast of Florida in 1991.
The average ex-vessel price was $US3.00/pound
US SCALLOP CULTURE AND ENHANCEMENT
45 Calico scallop
TOTAL LANDINGS (POUNDS X 10°)
8
76 77 78 79 ВО B1 82 ЕЗ 84 B5 ВВ 87 B8 89 90 01
EAR
FIG.8. Landings of calico scallops, Argopecten gib-
bus. Data from Blake and Moyer 1990; O'Bannon
1992b).
($US1.36/kg) of meats compared to $US1,13
(SUSD.5 | /kg) in 1990,
Since stocks of A. gibbus are annual, over-fish-
ing is not considered a problem, thus there are no
state or federal fishery management programs.
The fishery is totally dependent upon the natural
population and regulation of landings is governed
by a self-regulating association of industry mem-
bers. Fishing efforts are limited until at least 75%
of the stock at a particular location reaches a shell
height of at least 38mm, the point at which much
of the population will have undergone their first
spawning event. A second spawning is not
guaranteed and only takes place when environ-
mental conditions are optimal.
WEATHERVANE SCALLOP, PATINOPECTEN CAURINUS
This large, long-lived species reaching up to
с.10іпсћех (25cm) and 28 years of age (Hennick,
1973) occurs from Alaska to Oregon (Fig.9). It
requires 5—6 years to attain a shell height of
4inches (10cm) and reaches sexual maturity at
¢3inches (7em) shell height. Scallop meats are
large, similar in appearance to those of P. magel-
lanicus, and average counts are 10—40/pound (5—
I8/kg). Bourne (1990) reported that minor
landings of weathervane scallops occurred
sporadically along the coast of Washington until
the late 1950's with recorded landings for this
period (1935—1952) averaging about 3601
(320tonnes) (Cheney & Mumford, 1986), A small
fishery was developed in Alaska in 1967 and
landings have fluctuated widely (Fig.10; Table
5). Oregon landings for 1989-1992 were less than
500 pounds (200kg) per year; Washington land-
ings for the same penod ranged from 13,000
pounds (6tonnes) in 1989 to 6,700 pounds (3ton-
nes) in 1992, Alaska reported landings of 464,000
pounds (210tonnes) for 1989. These values do not
include confidential data; however, landings of P.
caurinus continue to fluctuate and represent à
small 96 of the US scallop fishery (NMFS).
Gear utilized ranges from old shrimp trawls to
typical east coast drag (Bourne,1990) and
methods of management vary. Alaska has had а
seasonal restriction (June 1- March 31) in some
areas, area closures and gear regulations. Many
regulations were designed to protect crab resour-
ces (Bourne, 1990). Minimum ring size on drags
must be 4inches (10cm) inside diameter (some
arcas permit use of a 3inch (7.5cm) ring) and
trawls have been eliminated from the legal gear
restrictions. Washington regulates its fishery bv
gearsize and mesh or ring size; Oregon by limite:
entry, gear and mesh or ring size; and California
management is by permits (Bourne, 1990),
PiNK SCALLOP, CHLAMYS RUBIDA
SPINY SCALLOP, CILAMYS HASTATA
Pink and spiny scallops are small and co-exist
in discontinuous populations along the US west
coast from Alaska to California (Fig.9); they are
often referred to as 'singing scallops'. They arc
slow-growing, rarely attaining shell heights
greater than 3.5inches (8em). These species sup-
port a small commercial fishery in Washington
and landings аге small (Fig. 10), The small size of
these scallops has encouraged a market for whole
scallops, often consumed steamed as one would
eat mussels or clams, This is a dangerous venture
given the paralytic shellfish toxins in the region
and ability of scallops to concentrate and retain
these Loxins for extended periods of lime (Shum-
way & Cembella, this memoir),
Fishing is by small drags or diving (Bourne,
1990) and the fishery is regulated by gear and
mesh size in Washington.
AQUACULTURE AND ENHANCEMENT
During 1920-1926, William Firth Wells car-
ried out some bivalve culture investigations
which he reported in his annual reports tothe New
York State Conservation Commission. Besides
propagating the easier oyster, Crassostrea vir-
ginica, he cultured quahogs, Mercenaria mer-
294
60°
Alaska
50°
Chlamys rubida (pink scallop)
MEMOIRS OF THE QUEENSLAND MUSEUM
Columbia
Chlamys hastata (spiny scallop)
40°
30°
160° 150°
Pacific Ocean
140° 130° 120°
FIG.9. Distribution of Pacific coast scallop species: weathervane scallop, Patinopecten caurinus; pink scallop,
Chlamys rubida; spiny scallop, Chlamys hastata. After Bourne 1990).
cenaria, soft clams, Mya arenaria, mussels,
Mytilus edulis and bay scallops, Argopecten ir-
radians (State of New York Conservation
Department,1969). Wells used a milk separator
to clarify his culture water and to collect larvae
from cultures for transfer. One of the earliest
species he cultured was the bay scallop. It was
perhaps the first bivalve cultured in the manner
similar to what we think of today as aquaculture
(late Joseph Glancy, pers. comm.).
Most scallop culture in the US now utilizes the
bay scallop, A. irradians irradians or A. irradians
concentricus. The species is characterized by
rapid growth, high fecundity and a high market
value (Castagna,1975; Castagna & Duggan,
1971,1972). The hatchery technology is well
known and successful manipulation of adult scal-
lops in the hatchery can provide a sexually mature
spawning stock throughout the year (Sastry &
Blake,1971; Barber & Blake,1981). A number of
companies have attempted to culture scallops but
have not been economically successful and there
US SCALLOP CULTURE AND ENHANCEMENT
Whole Weight (tonnes)
ü
Meat Weight (tonnes)
79 7? ТЕ 19 Mo 8! B2 а BÀ BS 5% BT
0а 4g 9 91
wal OREGON же
2000
1750
1500
(250
1020
720
50
30
76 77 78 70 50 81 00 83 HÀ 8 G5 B7 HÀ M5 30 3j 3c
YEAR
FIG.10. Landed value of weathervane scallops,
Patinopecten caurinus (Bourne, 1990; NMFS, pers.
comm. ).
is no profitable, private aquaculture industry for
bay scallops in the US (Rhodes, 1990; pers. obs.).
This species has been successfully cultured in
China (F. Zhang, K. Chew, pers. comm.) and the
product is being imported to the US. Recent un-
explained mortalities have been attributed to in-
sufficient genetic diversity and new broodstock
has been supplied by Canadian sources (Atlantic
Fish Farming, February 27,1993).
A few companies have been involved in enhan-
cement programs, also utilizing bay scallops. Per-
haps the most successful is carried out by the
Martha’s Vineyard Shellfish Group which is a
consortium of 5 towns (Chilmark, Gay Head, Oak
Bluffs, Tisbury and West Tisbury) on Martha’s
Vineyard off the coast of Massachusetts. This
group, using a number of federal and state grants,
built a solar-assisted hatchery to produce А. ir-
295
POUNDS OF MEATS ( x 1000.)
FIG.11. Commercial landings of bay scallops in New
York (from Tettelbach & Wenczel,1990).
radians irradians and clams, Mercenaria mer-
cenaria (Karney,I978). Their hatchery methods
are standard except that the seawater is partially
warmed in a passive solar system within the
solarium-type building. The post-set scallops are
held in an indoor, semi-closed nursery system
supplemented with cultured algae until the
juveniles are 3—5mm high, then moved out to a
small embayment in burlap bags with a brick
anchor and a plastic cola bottle inside the bag for
a float. Several hundred to a few thousand seed
are placed in each bag which is then anchored
over the submerged vegetation in the bay, This
allows the seed to grow to a size that offers
sanctuary from some predators before the bag rots
away allowing the juveniles to escape a few at a
time and spread into the vegetation (В.С, Karney
pers. comm.). Each township has legal jurisdic-
lion over its own shellfish waters, sale of harvest-
ing licenses and control of the harvest. Each
township supporting the hatchery buys seed at
about cost for replenishment or enhancement of
an area. The effect of scallop enhancement has
been to add a degree of stability to the harvest in
the area that 15 seeded (Karney,1978).
Another enhancement program was carried out
in the Long Island Sound area after heavy mor-
talities of native scallops caused by a picoplan-
kter, Aureococcus annophagefferens. Extensive
reseeding of hatchery-reared scallops was in-
itiated in the Peconic Bays by the Long Island
Green Seal Committee in 1986 (Tettelbach &
Wenczel,1993). In the following two years, seed
scallops (A. irradians) were purchased from a
number of hatcheries and released in selected
areas to enhance or replace the natural popula-
tions which were lost. The effects of this enhan-
cement effort Were not quantified in all areas, but
MEMOIRS OF THE QUEENSLAND MUSEUM
TABLE 6, Historic number of vessels, number of landings, Janded weight of shucked meats, price per pound,
exvessel value, landings per vessel, and exvessel value per vessel for the weathervane scallop fishery in Alaska
during 1967-1991. All data for 1967-1968, and prices and exvessel values for 1967-1975 and 1979 were taken
from Kaiser 1986); all other data were summarized from fish tickets. The 1991 data are preliminary. In years
when only one or two vessels participated in à fishery, the harvest statistics are confidential. (from Kruse et
al.,1992)
: EM
1970 7
wu | s |
1972 5
эз | 5s |. as |
1974 3 29 504,438
435,672
1976
1977
54 525,508
anumber of scientists involved in this experiment
initially believed the effects of the seed planting
were minimal (Bricelj et al.,1987; Tettelbach &
Wenczel,1993), Krause (1992), however,
showed about 25% of the scallops in the area were
survivors of those released. Subsequent reseeding
efforts were further hampered by the shell-boring
parasite, Polydora sp. and another “brown tide".
While enhancement efforts are encouraging, the
New York bay scallop fishery is precarious.
In the northeast there is some experimental
culture of the sea scallop, Placopecten magel-
lanicus at the hatchery on Beal Island, Maine. The
technology for culturing this species has already
been established in Canada; however aquaculture
of this species has not been attempted in the US
BHEIES Eu |e
Vessels Lundingy are Value ($)
чм А |
| 168 | 19 | ms | втв | oss | 125,678 | 88277 | 75036 |
EN NEN EN mea | a imes | CNET
| зт | 1440338 | — 100 — | 1440438 | 205,763 | 205763 |
| 60 | suas | ros | 9775709 | 186230 | 195542 |
| és | 61167034 | 115 | 1342089 | 233407 | 268418 |
ws | a [| s | азво | 140 |
——C st
1981 924.441 | aos | 3743986 | 51358 | 207999 |
€ n ERE EE | аз | кимыл зз ER
а ъс кын. 488 947,286 32,353 Omm. 881
1984 | 10 | в [| змат | 447 | 1142282 | 38982 | 174248 |
| ws | 9 | s | 67292 | 312 [| 2019551 | 71920 | 224395 |
| is | o | se | 6862 | 366 | 2498397 | 75847 | 277000 |
| 1з | 3 | ss | {mos | 338 | 1970685 | 145760 | 492671 |
1988 — 47 341.070 297,584
mr нагна DRY 1m m 5016724 | 165405 | 557,414
| gm | e | ma | ii3564 |
1,546,231 6,045,763 | 220,890 | 863,680
Landings Value ($)
(lbs) per per Vessel ,
Vessel
Confidential
655,769 168,146 218,590
609,94] 108,918 152,485
Confidential
Confidential
Confidential
3 68 1 ЭМ, 201 m 085 276,314
4,228,334 189,442
3.72 704,722
(Culliney,1974; Naidu & Cahill,1986; Beninger,
1987; Mallet, 1988). The present study plans to
test grow the scallops in near-bottom containers,
either bags or cages. Some of these will be placed
near salmon pens to see if the efflnents will en-
hance growth rates. Initial studies by Belle (pers.
comm.) indicate that increased growth rates can
be realized in oysters and sea scallops grown in
lantern nets suspended near salmon pens.
On the US Pacific coast, there has been some
previous interest in culturing the rock scallop,
Crassadoma gigantea (Jacobson, 1977; Leighton
& Phleger,1977,1981; Leighton, 1979a,b; Moni-
cal, 1980; Cary et al.,1981,1982) and there is an
experimental culture program in Washington for
this species (Chew, pers. comm.). A project was
US SCALLOP CULTURE AND ENHANCEMENT
TEW en Moning Bhais
= Crores бал
vimm Morir
E Fjernes ДТ
me / Уу VU уву пагы
4, їїш/ п! Маты авап
D 20 РА Мид туго
э d ару onan
=
=
5
= w
$ 3
Ss 2 T
ы; 9
6 '
2 a
4 ^
EM 7 50 H* 100 110 120 190 4%)
Миа Haight (mins)
FIG.12. Regression analyses for adductor weight vs,
shell height for Placopecten magellanicus from
various geographic locations (Schick et al. 1987).
initiated in Alaska in 1987 to determine the
feasibility of culturing weathervane scallops
utilizing natural spat sets. In the Washington state
hatchery, after preliminary culture experiments
on Pecten caurinus and C. gigantea (Olsen, 1981,
1983), C. gigantea was grown and released in an
atiempted enhancement program (Olscn,1984);
however, the numbers released were insufficient
to follow. Efforts have also been made to collect
juvenile pink and spiny scallops from natural spat
sets; however these species are too small and too
slow growing to support an economical culture
operation (Bourne, 1990),
Except for small, sporadic releases of Argopec-
ten or Crassadoma over the years, there are no
major scallop enhancement programs in the US.
Scallop culture (mainly research) is underway in
Maine, Massachusetts, New York and Virginia;
however, itcan hardly be considered a significant
or economically feasible activity.
ACKNOWLEDGEMENTS
We are indebted to Barbara O'Bannon, Robert
Morrill, and John Bishop of the National Marine
Fisheries Service for landings data and helpful
discussions. Thanks also to Gordon Kruse for
making Table 6 available. Jan Barter prepared the
tables and Jim Rollins drafted the figures.
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culture and fisheries.
STATE OF NEW YORK CONCERVATION
DEPARTMENT 1969, Early oyster culture inves-
tigations by the New York State Conservation
Commission 1920-1926.
TETTELBACH, S,T. & WENCZEL, P. In press.
Resecding cfforts and the status of bay scallop
populations in New York following the occur-
rence pf "brown tide” algal blooms Journal of
Shellfish Research.
REPRODUCTION AND RECRUITMENT IN THE DOUGHBOY SCALLOP, CHLAMYS
ASPERRIMUS, IN THE D'ENTRECASTEAUX CHANNEL, TASMANIA
WILL ZACHARIN
Zacharin, W. 1994 08 10: Reproduction and recruitment in the doughboy scallop, Chlamys
asperrimus, inthe D'Entrecasteaux Channel, Tasmania. Memoirs of the Queensland Museum
36(2); 299-306. Brisbane. ISSN 0079-8835,
Doughboy scallops in the D'Entrecasteaux Channel can grow to a shell height of 110mm.
Reproductive output in this population displays both temporal and spatial changes. The
highest gonosomatic index recorded was 45% for a doughboy of 105mm. The number of
mature eggs released in the 90-95mm size class was significantly different between two
annual peak spawnings and there is evidence for secondary or partial spawnings. Recruitment
monitoring through the deployment of spat collectors and sampling of the populations
suggests that hydrodynamic influences play an important role in recruitment success.
Will Zacharin, Sea Fisheries Division, Department of Primary Industry and Fisheries, GPO
Box 619F, Hobart 7001, Tasmania; 20 June 1994.
The doughboy scallop, Chlamys (Mima-
chlamys) asperrimus (Lamarck,1819), is an
abundant benthic bivalve found throughout
southern Australia. Large populations extend
over wide areas in Bass Strait, and a commercial
and recreational dredge fishery for the species has
operated irregularly in the D'Entrecasteaux
Channel in southeastern Tasmania since the
1930's (Perrin & Croome,1988). An annual
recreational dive fishery in the D'Entrecasteaux
Channel is now the only remaining fishery.
For such a prominent member of the southern
Australian benthic community, surprisingly little
information exists in the scientific ljterature on
the life history of the species. Larval and juvenile
development of the doughboy were studied by
Rose & Dix (1984); observations on epizoic
sponge associations with the doughboy have been
reported by Pitcher(1981) and Pitcher & Butler
(1987), and some factors affecting mortality were
described by Chernoff (1987). However, no
studies have been conducted on growth,
reproduction or population dynamics.
This study describes the reproduction and
recruitment of the doughboy scallop in the D’-
Entrecasteaux Channel, a semi- enclosed inshore
waterway in southern Tasmania (Fig. 1).
MATERIALS AND METHODS
A sample of 10-50 doughboys was collected
from the same population in Simpson's Bay at 14
day intervals over 28 months (1 July 1988-27
November 1990), In the laboratory the animals
were measured (shell height to the nearest
0.1 mm) and total somatic and gonad tissue were
weighted to the nearest 0.1g. Sex was determined
according to colour of the gonad, males being
white and females orange. Gonosomatic Index
(GST) was calculated as a ratio of gonad weight
to somatic tissue weight. A significant decrease
in the index was considered to be an indication of
spawning (Dredge,1981; Sause et al, 1987; West,
1990), The terminology of stages in the gonad
reproductive cycle was based on that of Pecren
fumatus (Sause et al.,1987), Chlamys varia
(Shafee & Lucas.1980) and Amwsium ballori
(Dredge, 1981).
A fecundity index was developed using an in-
direct method, in which the difference in gonad
weight of mature female scallops immediately
pror to spawning and after spawning was calcu-
lated. This weight loss on spawning can be used
as an index of the number of ova released from
the gonad. The underlying assumptions are that
mature ova prior to spawning have the same mass
from year to year, and ova mass is the same across
all size classes.
Regular surveys of doughboy populations in
the D’Entrecasteaux Channel have been con-
ducted by the author since 1985 to monitor
recruitment. Between 1985 and 1988, 110-119
random stations within each statistical area were
sampled using a 2.5m wide tnothed scallop
dredge (Zacharin, 1986,1987,] 988). Since 1989,
scallop surveys have been conducted by diving,
to more accurately sample doughboys in the size
range 30—40mm (1+ animals) (Zachann, 19912,b;
Zacharin et al., 1990).
As scallops are usually distributed at low den-
sities over Jarge areas and at high densities form-
ing ‘commercial beds” over small areas, sampling
300 MEMOIRS OF THE QUEENSLAND MUSEUM
DERWENT
Oyster Cove &
Barnes Bay
ISLAND’
Houn River
Simpsons Вау
Adventure Bay
SOUTH
BRUN Y
Банда)
Island
ISLAND
FIG.1. D'Entrecasteaux Channel as divided into statistical areas by Fairbridge (1953) for conducting scallop
surveys. The same boundaries were used for the dredge and dive surveys between 1986 and 1992. (from Perrin
& Croome, 1988)
DOUGHBOY SCALLOP, TASMANIA
28 Sept &%
WRI
4
16 Nov 9f)
/
13 Gel 88
7 Dec 88
4
3 Noy 80
MEAN GONOSOMATIC INDEX
I
5 fan 90
d read ї
n 74 161 3847 442 539 765 82h
DAYS
FIG.2. Seasonal changes in mean gonosomalic index
in the female doughboy scallop from the D'-
Entrecasteaux Channel. (Error bars =one standard
deviation.).
technique must compensate for this fact. To ad-
just for this pattern of distribution, diver surveys
were conducted using the following procedure. A
number of random sampling points were dis-
tributed over an area to give an indication of
scallop distribution. Further non-random sam-
pling points were chosen based on previous catch
history of the area and from reported sightings by
divers, Ateach site a 100m transect line weighted
with lead and buoyed at each end was deployed
parallel to the current, Two divers swam along the
transect collecting all scallops within 1m of the
weighted line. It is important to deploy the line
301
15 Sepi 88
2 ext RO
pA
7 Dec КА
13 Oct SH
MEAN GONOSOMATIC INDEX
"4
3 Jan 88
n 105 324 450 728 82h
DAYS
FIG.3. Seasonal changes in mean gonosomatic index
in the male doughboy scallop from D’ Entrecasteaux
Channel. (Error bars=one standard deviation.)
with the current and to swim with the current, as
any scallops disturbed by the deployment of the
line may move. As scallops tend to swim off the
bottom and then free-fall to the substrate the
majority are more likely to remain in the transect
area if deployment is parallel to the current. The
data were assembled as both total size frequen-
cies for the whole of the channel area and as size
frequencies of scallops in the various statistical
areas.
Spat collection was conducted using small
orange coloured onion bags with dark monofila-
ment mesh filling as a settlement substrate. The
TABLE 1. Description of gonads and the histological condition of the various stages in the annual reproductive
cycle of the scallop, Chlamys asperrimus from the D'Entrecasteaux Channel, Tasmania.
Female
Male
(1) Resting Gonad small, flat and yellow brown. Composed of loose connective tissue. Intestinal loop visible.
Ciliated ducts present
(2) Early Slight increase in gonad size. Follicles with primary oogonia or spermatogonia. Clear differentiation of
development male and female gonads. Intestinal loop not visible.
(3) Late Gonad increased in volume, tip being tapered.
development
Gonad orange.
(4) Mature
(5) Spawning
Loss of gonad colour.
Gonad volume large with rounded np. Little connective tissue.
Follicle packed with mature irregular polygonal Large number of spermatozoa. Follicles tightly
oocytes. acked.
Free space in the centre of the follicles as gametes are expelled. Appearance of more connective tissue.
Gonad white.
(6) Spent
Follicles nearly empty of all gametes. Increase in connective tissue. Phagocytes predominate.
302
(19)
(1211)
Gonad Weight (g)
55
60 =
5
(
75
a0
85 7].
E
5
100
105
(10
Shell Height (mm)
FIG.4, Fecundity index shown as a relationship be-
tween shell height (5mm intervals) and gonad weight
for mature samples (A) collected on 15 and 28 Sep-
tember 1988 and immediate post-spawning samples
(B) on 13 and 20 October 1988. Number of scallops
shown in brackets.
first spat collectors were deployed on 18 Septem-
ber 1988 at various locations throughout the
channelarea. Sites were selected where tidal flow
was greater around prominent headlands and is-
lands. Collectors were observed each month to
assess the intensity of spat settlement.
RESULTS
REPRODUCTION
Six distinct stages of development were recog-
nised (Table 1). During late summer to autumn
(January-March) gonads were completely spent
and appeared to be in a ‘resting phase’. Accurate
macroscopic identification of sex for the majority
of individuals during the ‘resting phase’ proved
10 be impossible.
Fortnightly changes in mean GSI of females
and males (Figs 2,3) are interpreted as increases
TABLE 2, Results from spat collectors deployed ad-
jacentto Huon Island in the D' Entrecasteaux Channel
(statistical area 9) during 1988/89,
Standard
deviation
316
| 28/2/89 230 10.43 1.97
21/3/89 226-306 12.65 2.69 |
MEMOIRS OF THE QUEENSLAND MUSEUM
(1)
Пипа Weight (2l
=
(8) 119001}
i (B)
© d c чї € л c V1 c Un
wy б MD p or o6 c с б
[ШЇ]
105
110
Shell Height (mm)
FIG.5. Fecundity index shown as a relationship be-
tween shell height (5mm intervals) and gonad weight
from mature samples (A) collected on 4 and 12 Oc-
tober 1989 and immediate post-spawning samples (B)
on 2 16 November 1989, Number of scallops shown
in brackets.
in gonad weight due to follicular development
and production of gametes; rapid decrease in
gonad weight in September-October was indica-
tive of spawning. The differences in gonad
weights (being an index of ova number) for
grouped samples (Smm) indicated a significant
increase in ova number for the older and larger
scallops (Figs 4,5). With the exception of rare
large doughboys, gonad weight increased with
size and peaked in the 90-95mm size class. Male
GSI peaked earlier than female, and males ap-
peared to commence releasing sperm earlier than
females shed ova (Figs 2,3). GSI peaked earlier
in 1988 (September) than in 1989 (October). The
index of fecundity was significantly higher in
1988 with the average gonad weight of the 90-
95mm size class being 3896 higher than in 1989
(t-test, Р<0.02). Gonad weight loss on spawning
in 1988 for this size class was 63.1296 of total
gonad weight compared to 56.83% in 1989 (Figs
4.5). A significant decrease in gonad weight (sug-
gestive of spawning) was observed between Sep-
tember and December in each year.
In both years there was a second rapid decline
in gonad weight in late December-early January.
This has been interpreted as being indicative of
partial spawning. Data obtained from spat collec-
tors supports this concept. It is not known what
percentage of gametes released through earlier
partial spawnings or late spawnings are com-
petent; or their contribution to recruitment. How-
ever, collectors placed ata number of locations in
DOUGHBOY SCALLOP, TASMANIA
FREQUENCY
199]
110
6
7
bI
)
100
SHELL HEIGHT (mm)
303
2504 1990
FIG.6. Frequency histograms of all doughboy scallops measured from all sites sampled during the 1989, 1990
and 1991 dive surveys.
the D'Entrecasteaux Channel between Septem-
ber and April suggest that minor settlement oc-
curs over a number of months, but there is one
major event (Table 2). The highest spat numbers
(«5mm), in December, suggest the major Sep-
tember/October spawning contributes to greater
spat settlement.
Sex ratio for all samples collected was 1:1.
There was no change in sex ratio observed be-
tween different ages or shell height.
RECRUITMENT
Fig.6 shows the change in size frequency from
1989 to 1991. As the diver surveys were con-
ducted between March and April each year, an
index of potential recruitment was represented by
numbers of the 1+ year class (30—40mm size
range). Both survey results (Zacharin 1989, 1991,
Zacharin er al. 1990) and observations of spat
settlement indicate that there was strong settle-
ment in 1988 and 1990. Size frequency his-
tograms (Fig.7) demonstrate the spatial
patchiness of scallop settlement and subsequent
recruitment by the relative abundance of 30-
40mm scallops. In statistical areas 7, 8 and 9 such
scallops were relatively abundant, particularly in
1991. In areas 6 and 10, 30-40mm scallops were
rare except in the 1991 samples, whereas area 11
supported few recruits throughout the entire
study period. The remaining seven statistical
areas (Fig.l) were not sampled with sufficient
regularity to give meaningful data.
DISCUSSION
Sustainable management of a scallop fishery is
dependent in part on an understanding of the
reproductive cycle and environmental influences
that may change or alter the timing and frequency
of spawning. An important objective of the
fishery manager is to identify the minimum size
and age at first maturity, to reduce the potential
for recruitment overfishing. Knowledge of the
reproductive cycle is also important in determin-
ing when, and to a lesser extent, where recruit-
ment to the fishery may occur (Orensanz, 1986).
304
Area 6
MEMOIRS OF THE QUEENSLAND MUSEUM
FREQUENCY
[1 T]
> ©
26 c
ре
e
SHELL HEIGHT (mm)
L] 1989
[|
1990 1991
FIG. 7. Size frequency histograms by statistical area for doughboy scallop populations from dive surveys
conducted in 1989, 1990 and 1991. Areas 10 and 11 have data from 1990 and 1991 only, while Area 9 has data
from 1989 and 1991 only.
C. asperrimus is a synchronous spawner, as is
P. fumatus (Sause et al.,1987) and E. bifrons (Dix
& Sjardin,1975). However males matured and
released sperm earlier than females. Gonads
began early development in late March-early
April. Maturation continued through the winter
months and a major spawning event occurred in
late September-mid-October. À minor spawning
event was observed in December; however, the
significant decrease in GSI at this time may have
been a consequence of oocyte lysis and reabsorp-
tion (Zacharin pers. obs.). Rose & Dix (1984)
DOUGHBOY SCALLOP, TASMANIA
collected zygotes from individuals in the D'-
Entrecasteaux Channel during September/Oc-
tober in their study of the larvae of C. asperrimus,
which is consistent with the results of this study.
Fecundity generally increased with shell height
and age and peaked in the 90-95mm size class,
Few doughboys larger than 95mm were found, Of
the two located, one 101mm individual found in
1989 had the highest gonad weight recorded
(16.4g).
The results illustrate the need fo monilor
populations over a number of seasons to establish
the timing, frequency and level of ova release
during spawning. The major spawning in 1988
occurred between 15 September and 20 October
with the maximum mean GSI being 37.72% on
15 September. In 1989 the major spawning oc-
curred four weeks later between 4 October and 16
November on the basis of GSI changes. Maxi-
mum gonad index is reached 2-3 weeks prior ta
spawning, and some gamete ‘leakage’ occurs
prior to the main spawning event, This was
revealed by early spat settlement in the collectors,
Gonad weight loss was used as 2 measure of
fecundity, as the number of ova released in any
year may widely fluctuate. A count of total ova
number, as is performed in many fecundity
studies, may not have highlighted this difference.
Total ova number released annually is preferable
to the number of mature ova contained in the
ovary. Research into stock/recruil relationships
may be easier to interpret if the former and not
the latter measure is more widely used,
Spat collection was an important process used
to validate identification of bath the peak spawn-
ing period and secondary or minor spawning
events. During the two year period 1988/89—
1989/90, highest spat numbers were recorded in
December, with shell height frequency his-
tograms indicating a further minor settlement in
February. Spat «2mm shell height were ohserved
in spat collectors during November-March, in-
dicating some partial spawning or ‘leakage’ of
gametes at a low level over a 5 month period. This
gamete leakage has been reported for a number
of other scallop species (Brand et al., 1980; Cioc-
c0,1991; Ноте & Cropp,1987; Sause etal., 1987;
Wolff, 1988).
Recruitment in the D'Entrecasteaux Channel
region has been spatially and temporally erratic.
Settlement of juveniles was high in both 1988 and
1990 with the highest number of recruits ob-
served in 1990. Models of larval advection show
that the strength and direction of wind at the Time
of spawning is an important determining factor in
305
the distribution of scallop larvae. (Butman, 1987;
Orensanz et al,,1991; Young et в1.,1992). This is
well illustrated by the spatial changes in spal
settlement and distribution of juvenile scallops in
the D’Entrecasteaux Channel,
LITERATURE CITED
BRAND, A.R, PAUL, 1D. & HOOGESTER, J.N,
1980. Spat settlement of scallops Chlamys oper-
cularis (L.) and Pecten maximus (L.) on artificial
collectors. Journal of the Marine Biological As-
sociation uf the United Kingdom 60: 379-390.
BUTMAN, C.A 1987, Larval settlement of soft-sedi-
ment invertebrates: the spatial scales of pattern
explained by active habitat selection and the
emerging role of hydrodynamic processes.
Oceanography and Marine Biology Annua!
Review 25: 113-165,
CHERNOFF, H. 1987, Factors affecting mortality of
the scallop Chlamys asperrima (Lamarck) and its
epizoic sponges tn South Australian waters, Jour-
nal of Experimental Marine Biology and Ecology
109: 155-171.
CIOCCO, N.F 1991, Differences in individual growth
rate among scallop (Chlamys techuelcha (d'Orb))
populations from San Jose Gulf (Argentina).
Fisheries Research 12: 31-42,
DIX, T.G, & SJARDIN, MJ, 1975. Larvae of the
commercial scallop (Pecten meridionalis) from
Tasmania, Australia, Australian Journal of Marine
and Freshwater Research 26: 109-112.
DREDGE, M.C.L. 1981. Reproductive biology of the
saucer scallop Amusium japonicum balloti (Ber-
nardi) jn central Queensland waters. Australian
Joumal of Marine and Freshwater Research 32-
775-787,
HORTLE, M.E & CROPP, D.A. 1987. Settlement of
Ihe commercial scallop, Pecten fumatus (Reeve)
1855, on anificial collectors in easter Tasmania.
Aquaculture 66: 79-95,
DRENSANZ, J.M. 1986. Size, environment and den-
sity: the regulation of @ scallop stock and ils
management implications, Canadian Special Pub-
lication an Fisheries and Aquatic Science 92: 195-
227,
ORENSANZ, J.M., PARMA, A.N. & IRIBARNE,
0.0, 1991, Population dynamics and manage-
ment of natural stocks. Pp.625-714. In S.E. Shum-
way, (ed), ‘Scallops: biology, ecology and
aquaculture’. (Elsevier. New York).
PERRIN, К.А. & CROOME, R.L. 1988. The D'-
Entrecasteaux Channel scallop fishery: Tts past
and possible future. Papers and Proceedings of the
Royal Suciety of Tasmania 121: 179—197,
PITCHER. С.К. 1981, Same mutualistic aspects of the
ecology of the scallop Chlamys asperrimus
(Lamarck) and its Epizoic sponges. BSc Hons
Thesis, University of Adeliade,(Unpubdl.),
PITCHER, С.Е, & BUTLER, A.J, 1987, Predation hy
306
asteriods, escape response, and morphometrics of
scallops with epizoic sponges. Journal of Ex-
perimental Marine Biology and Ecology 112:
233-249.
ROSE, R.A. & DIX, D.G. 1984. Larval and juvenile
development of the doughboy scallop, Chlamys
(Chlamys) asperrimus (Lamarck) (Mollusca:Pec-
tinidae). Australian Journal of Marine and Fresh-
water Research 35: 315-323,
SAUSE, B.L., GWYTHER, D., HANNA, PJ. &
O'CONNOR, N.A. 1987. Evidence for winter-
spring spawning of the scallop Pecten alba (Tate)
in Port Phillip Bay, Victoria. Australian Journal of
Marine and Freshwater Research 38: 329-337.
SHAFEE, M.S. & LUCAS, A. 1980. Quantitative
studies of the reproduction of black scallop,
Chlamys varia (L.) from Lanveoc area, Bay of
Brest, Journal of Experimental Marine Biology
and Ecology 42: 171—186.
WEST, G. 1990. Methods of assessing ovarian develop-
ment in fishes : a review. Australian Journal of
Marine and Freshwater Research 41: 199— 222.
WOLFF, M. 1988. Spawning and recruitment in the
Peruvian scallop Argopecten purpuratus. Marine
Ecology Program Series 42: 213-217.
MEMOIRS OF THE QUEENSLAND MUSEUM
YOUNG, P., MCLOUGHLIN, R.J. & MARTIN, R.B.
1992, Scallop (Pecten fumatus) settlement in Bass
Strait, Australia. Journal of Shellfish Research 11:
315-323.
ZACHARIN, W. 1986. D'Entrecasteaux Channel scal-
lop survey, 1986. Tasmanian Department of Sea
Fisheries Technical Report 9: 1-17.
ZACHARIN, W. 1987. D'Entrecasteaux Channel scal-
lop survey, 1987. Tasmanian Department of Sea
Fisheries Technical Report 16: 1—17.
ZACHARIN, W. 1988. D'Entrecasteaux Channel scal-
lop survey, 1988. Tasmanian Department of Sea
Fisheries Technical Report 27: 1—16.
ZACHARIN, W. 1991a. Tasmanian Zone scallop sur-
vey, 1990. Tasmanian Department of Sea
Fisheries Technical Report 42: 1-11.
ZACHARIN, W. 1991b. D'Entrecasteaux Channel
scallop survey, 1991. Tasmanian Department of
Sea Fisheries Technical Report 45: 1—16.
ZACHARIN, W., GREEN, R. & WATERWORTH, C.
1990. Estimated abundance of doughboy, queen
and commercial scallop stocks in the D'-
Entrecasteaux Channel, Tasmania 1989. Tas-
manian Department of Sea Fisheries Technical
Report 40: 1-17.
SUSTAINABLE MANAGEMENT OP BASS STRAIT SCALLOPS
RICHARD J. MCLOUGHLIN
McLoughlin, R.J, 1994 08 10: Sustainable management of Bass Strait scallops. Memoirs of
the Queensland Museum 36(2); 307-314, Brisbane. ISSN 0079-8835.
The history of scallop fishing in Bass Strait, ils management and research is briefly reviewed.
The extent of the known biology and ecology of the species is discussed in relation to the
current management of the fishery, and an assessment of {һе current policy is prece
Growth and recruitment overfishing as it relates to this fishery is discussed ii 1
ght of the
"two spawnings' criterion underpinning the current management plan.
Richard McLoughlin, CSIRO Division of Fisheries, CSIRO Marine Laboratories. G.P.O.
Box 1338, Hobart, Tasmania 7001; present address: Department of Primary Industry and
Fisheries, G.P.O. Box 619F, Hobart, Tasmania 7001; 9 March, 1994,
Large concentrations of the ‘commercial’ scal-
lop, Pecten fumatus, were located off Lakes
Entrance in Bass Strait іп 1970, Research surveys
sponsored by the Tasmanian Government be-
tween 1971 and 1973 located promising scallop
beds along northern Tasmania, These were first
exploited in 1973, a year thal saw a resurgence of
fishing activity in Port Phillip Bay. Discovery of
major new beds off the Furneaux Island Group in
eastern Bass Strait in the late 1970s sparked off a
penod of rapid expansion in the scallop industry.
Fishing activity in the region increased dramati-
cally, and total Jandings reached a record high in
1982-83 when the total catch (live weight) ap-
proached 12,000 tonnes and the number of ves-
sels participating tripled in two years to 231
vessels (Young & Martin,1989; Zacharin, 1990).
By 1985, the main beds in Bass Strait were
depleted and the decline in landings was just as
dramatic as the rise, Banks Strait, the last major
scallop bed in Bass Strait, was fished out during
1986. The Tasmanian zone of southem Bass
Strait was closed to scallop fishing following the
1987 season, and surveys have since shown that
there has been little subsequent recruitment
(Zacharin,1987,1989; McLoughlin et al.,1988;
Martin et aJ., 1989; Martin, 1990).
Over this 20 year period, few, if any, commer-
cially fished scallop beds have supported ex-
ploitation for more than 2 consecutive seasons.
Few of these beds consisted of scallops compris-
ing more than ] or 2 year classes. The conclusion
is that effort and capacity in this fishery has built
up to the point that single recruitment events,
resulting in discrete scallop beds of single year
classes, are quickly fished out as the majority of
the bed reaches a size at which they become
commercially viable to land (McLoughlin et al
1991). These beds do not, in general, appear to
regenerate or provide additional year classes of
scallops in the time frame of the current fishery
c. 20 years.
MANAGEMENT AND RESEARCH
Zacharin (1990) and Gwyther (1990) describe:
the management of Bass Strait scallops from both
Tasmanian and Victorian perspectives. Present
management resulted from concerns by State and
Commonwealth Governments (and industry) fol-
lowing expansion during 1979-1983, The Bass
Strait Interim Management Regime of November
1983 saw 97 Victorian and 134 Tasmanian based
vessels gaining access to the whole Bass Strait
fishery (i.c. their respective state waters and the
Commonwealth controlled central zone greater
than 20 nm from the coastline of the two states
(Fig.1)).
The Commonwealth Government then estah-
lished the Bass Strait Scallop Task Force
(BSSTF), consisting of government and industry
representatives, whose brief was to develop a
management plan that: 1, Effectively utilised the
resource; 2. Was acceptable to all parties; and 3.
Was legally enforceable. The Task Force was not
able to develop а management plan agreeable to
all parties, and the final recommendation
presented 10 the 1985 meeting of the Australian
Fisheries Council was to effect a high degree of
separation between the Tasmanian and Victorian
based fleets. Access to the Commonwealth con-
trolled central zone was restricted to scallop ves-
sels that qualified either for a Tasmanian or
Victorian state license and that had an endorse-
ment of their Commonwealth Fishing Boat
Licence. The separation was finalised under Off-
shore Constitutional Settlement agreements be-
tween the commonwealth and state governments
308
FIG.], Map of management zònes for Bass Strait seal-
lops. The boundaries for the two states lie 20 nautical
miles offshore, with the islands belonging to Tas-
mania state walers.
in June 1986 (Zacharin, 1990). This management
plan had no biological or objective fishery
management principles as its basis,
Apart from limitations on entry into the central
zone fishery, no other effort or catch control
regulations were imposed until June 1990], when
the then Commonwealth Minister for Primary
Industries and Energy, Hon. John Kerin MP, an-
nounced closure of the Bass Strait central zone ta
all scallop fishing. In his media release the Min-
ister stated that he had no option but to close the
area until there was clear evidence that stocks had
recovered to a level which would support a sus-
tained and substantial commercial fishery. The
Commonwealth decision was (apparently)
prompted by two considerations: 1. The CSIRO
recommendation in carly 1989 that no fishing be
allowed on any of the few remaining beds in Bass
Strait until stocks recovered (Fishing Industry
Research and Development Corporation grant no
1985/83 final report); and 2, Reports of limited
fishing on beds of apparently immature scallops
in the central zone by Victorian fishermen.
Only one major study of the biology, ecology
and fishery for scallops in Bass Strait has been
carried our (FIRDC 1985/83), It was the final
report of this study to the funding body that
included a recommendation for a ‘two
spawnings' criterion, Regional surveys during
the CSIRO study indicated severe stock depletion
and a lack of recruitment (McLoughlin et al.,
1988; Martin et àl., 1989), The research indicated
that spawning age stocks had fallen to such à low
leve] that failure to protect existing beds could
preclude recovery of scallop stocks in Bass Strait
for some years.
The CSIRO study also recommended thai a
MEMOIRS OF THE QUEENSLAND MUSEUM
high priority should be to regular monitoring of
the distribution and abundance of recruits and the
size and condition of scallops on the few remain-
ing beds. However this monitoring work was not
carried out as it was not funded. The last survey
of scallop stocks in the region was thal carried out
by CSIRO in May-June 1988. The decision by
the Commonwealth to close the zone in 1990 was
made in response to what appeared to be the
imminent resumption of unregulated fishing fol-
lowing the collapse 3 years earlier, and concern
as to the impact this might have on the recovery
of scallop stocks, A bed of apparently immature
scallops found near Deal Island in 1990 was
surveyed in June of the same year and, despite
some discussion based on interpretation of modes
in the length frequency data, they were assessed
by CSIRO and the Bureau of Rural Resources
(BRR) as being predominately composed of 1+
year class scallops with a minor (6%) 2+ year
class component (Martin 1990). It was this bed of
scallops that was at risk of a resumption of un-
regulated fishing, and which prompted mini-
sterial action.
In December 1990, the Commonwealth, Tas-
manian and Victorian Primary Industry Ministers
jointly announced a пем management plan for the
central zone scallop fishery; it represented a fun-
damental change in management philosophy
(Anon. 1991). Two aspects of the new arrange-
ments were: 1, that the Commonwealth wished to
work towards handing over management to the
two states, with an agreed jurisdiction line across
the Strait for purposes of state fisheries ad-
ministration; and 2, that opening of the central
zone would be dependent upon the "presence of
commercial beds that have had the opportunity to
spawn twice’.
OVERFISHING AND EXPLOITATION
STRATEGIES
By the end of the 1987 season the fishery hud
collapsed. The tolal catch by Tasmanian vessels
in 1987 was less than 500 tonnes, representing a
95% drop in annual landings in six years
(Zacharin, 1990) with Victorian vessels landing
220 tonnes, a 90% drop in catches over the same
period. CPUE for the sume period dropped to
13% of 1982/83 levels; industry and managers
accepted that overfishing was occurring,
Although generally applicable to all fish stocks,
Sinclair et ul. (1985) distinguished two types of
overfishing of scallops; ‘recruitment overfishing’
and ‘growth overfishing’. The first concept X. d
MANAGEMENT OF BASS STRAIT SCALLOP FISHERY 309
Pecten fumalus — Bass Strait Calendar
Year class n (м 1 M
Age (months) ü 7-10 12 31~34
Month Oct ™ Noy ——s June-Sept —- Nov Juno-Sept —®=| Nov —— Juno-Sept — Nov
i = =
@
Even E iE а d
a kl H ои
3 E р
i i
Gonads mamre for Gonads mature for
first ome. Number second time. Eggs
ate of eggs produced is ripen and spawning
T small amid these may occurs, This is the
not be spawricd
‘Harvesting at this Harvesting at this -Harvesting at this
point will remove point will remove point will remove
Effect of fishing most or all of year most or all of year
season starting class before they class before they
in April each year cun compleie their can complete their
2nd major spawning. Ard major spawning,
"The indirect effects ‘The indirect effects
of fishing may kill of fishing may kill
those scallops not those scallops not
caught by dredging, caught by dredging. Caught by dredging.
tst major spawning.
Gonads mature for
the third time. Eggs
ripen and spawning
occurs. This із the
2nd major spawning.
Gonads mature for
the fourth tine. Begs
реп atid spawning
occurs. This is the
Ard major spawning,
FIG,2. Biological calendar of scallop reproduction and fishing practice in Bass Strait scallops
ves the self-reproducing capacity of the popula-
tion and describes a level of fishing that begins to
limit the ability of the mature spawning popula-
tion to effectively provide adequate future
recruitment - in this case, adequate in terms of
providing a commercial fishery. The second con-
cept is relatively better understood, and describes
fishing at a size or age at which the maximum
yield in weight (or dollars if economic informa-
tion Is available) is not realized; that is, fishing
the resource at à ‘small’ size when à larger size
would provide better returns.
Having recognised the need for active manage-
ment in Bass Strait, a number of strategies are
worthy of consideration, including maximising
yield from individual beds or populations. One
approach to maximising yield from a population
where annual recruitment to the same population
can be ignored is to calculate the annual balance
between growth of the scallops and removal from
the population by death (Mohn,1986). This ap-
proach is relatively easy to evaluate given infor-
mation about growth rates, mortality rates at age
and the effective gear selectivity rates for each
age, The problem lies in the assumptions of the
'yield-per-recruit models used to derive this in-
formation, as they do not generally consider
either environmental or fishery related variations
in annual recruitment.
For example, the yield-per-recruit approach
will generate the same advice on optimal fishing
strategies whether or not recruitment overfishing
is occurring, and yet it would be critical for a
fishery manager to modify the strategies for
management if such recruitment overfishing was
occurring. Martin et al. (1990) examined the
problems of simple yield-per-recruit manage-
ment strategies for the Bass Strait scallop fishery,
and concluded that they would generally provide
poor results. These problems were further dis-
cussed by Young & Martin (1989).
It is still not certain whether recruitment of
scallops in Bass Strait is dependent upon supply
of larvae from nearby beds. However, regular
spat monitoring over two years at six sites in Bass
Strait, and advection modelling of larval trajec-
tories with real wind and tide data using a verified
circulation model (Fandry,1983), showed that
larvae are conserved within Bass Strait in all but
the windiest years (Young et al.,1992). A posi-
tive relationship between commercial catch rates
and spatfall in the same 1° square in Bass Strail
310
also provides some evidence of a stock recruit-
ment relationship (Young et al., 1990). Thus scal-
lop stocks are probably self-sustaining in Bass
Strait and a viable spawning stock should be
maintained. Although a minimum stock level
cannot be defined, a conservative approach to
prevent recruitment overfishing should he incor-
porated into the management plan. Several
authors have identified the possibility of recrujt-
ment overfishing in Bass Strait (McLoughlin ct
al,,1988; Martin et al,,1989; Zacharin.1 990;
Young & Martin, 1989); the Bass Strait scallop
management plan initiated in 1991 aims to avoid
this problem,
AGE AND FECUNDITY
A biological ‘calendar’ of scallop reproduction
and fishing (Fig. 2) shows that to achieve two
major spawnings, scallops must be in their third
year of growth, Fecundity of scallops of various
ages and from various areas of Bass Strail was
first determined by CSIRO during their 3 year
research program (Martin et al.,1990). While
fecundity was found to be. variable, there ap-
peared a relationship of size (age) and egg
production, with 3+ year class scallops shedding
3-5 times as many eggs as 1+ scallops. These 3+
scallops were 75-85mm shell height. Although a
linear age/fecundity schedule for Bass Strait scal-
lops is drawn for simplicity (Fig.2) it is probable
that (he. relationship is non-linear, reaching an
asymptote at some value approximate to maxi-
mum size of this species at around 140mm shell
height.
STOCK MANAGEMENT
Despite the inherent problems with the assump-
tions underlying yicld-per-recruit models, it is
useful to consider an example of growth overfish-
ing modelled for a hypothetical Bass Strait scal-
lop bed. Typically, the model used is an analytical
yield model developed for exploited fish popula-
tions (е.р., Beverton & Holt,1957). Sinclair et al.
(1985) for example, used such a model modified
for use on scallops to estimate yield as a function
of fishing effort and gear selectivity-at-age,
where the annual catch from the population under
a given fishing strategy equals the catch that can
be taken from a single cohort throughout its life
under the same strategy. The yield is maximised
by maximising the following relationship for a
single cohort,
MEMOIRS OF THE QUEENSLAND MUSEUM
'
Y=) Р. MW. dt (1)
rl
where Y is the annual yield, Pt 15 the instan-
taneous fishing mortality at time t, Nt is the
abundance of the cohort at time t, and Wt is the
meat weight in grams of the individual scallops
at time t. The model assumes that fishing takes
place on the same cohort over a number of years,
and the management aim in this case is (0 larget
fishing on the age class (or time) when the yield
is maximised, Sinclair et al, (1985) used this
approach to examine the effect of yarying
management strategies in the Canadian fishery
for the sea scallop Placapecten magellanicus,
where up to 13 year classes may be present and
fishing does not usually target on scallops until
they are 5-6 years old. Their analysis showed
clear overall yield benefits in targeting older year
classes.
The same situation of multiple year classes and
annual recruitment to beds does not occur in the
Bass Strait fishery. Here, scallop beds are
generally composed of a single dominant year
class, are fished to ‘extinction’ in the same year
as fishing commences, and very little survival
occurs into the next year, The model does notthen
have to formally take into account fishing mor-
tality in each year, since typically it is always in
excess of the capacity of the bed to survive into
the next year (McLoughlin et al.,1991). The
тое! reduces to calculating the yield in each
year from a given age/size structure, exploitation
rate (E) and stock size (N), traded off against an
annua] natural mortality (M). An equation to cal-
culate yield at time t thus reduces to:
Y E 7 № ‘ W, (2),
while the number of scallops available for cap-
ture in each year is
Ni+1= N(1-M) (3),
This has been modelled for three management
strategies, using data from average meat weights
at age for Bass Strait scallops and a ‘hypo-
thetical’ scallop population, where:
Nt: Although used only as an example,
Zacharin (pers. comm. 1990) calculated the stock
size available for capture at a bed near Deal Island
as c.26 million scallops, assuming an overall 30%
dredge efficiency during the survey of June 1990
(Маліп, 1990; McLoughlin et al.,1991), Assum-
MANAGEMENT OF BASS STRAIT SCALLOP FISHERY
TABLE 1, Analytical (meat) yield model results for
three management strategies, where (a) is scallops
fished from age 1+ onwards, (b) is scallops fished
from age 2+ onwards, (c) is scallops fished from age
3+ onwards, and (d) is scallops fished from age |+
onwards with a selectivity factor of 50% for the 1+
scallops
[pe
Fa EXEC M BOUM
a 1 | | 235 | 3 | 1809 | 9
| 1.5 5 — a ms
| a Es Toe T8 7]
|
(b) 2,2} n2 jiss |
з [ 109 |
(d) modified (50%) selectivity for year 1 сано.
ing а realistic cohort age on this bed (from length
frequency data), this has been converted to an age
structured population of 23.5 million 1+ year
scallops (90.3%), 1.5 million 2+ year scallops
(5.7%) and 1 million 3+ year scallops (4%).
Wt: While average meat weights vary sig-
nificantly in Bass Strait, particularly for the King
Island beds, yields are given for average meat
weights for age/size classes encountered in
spawning condition,
M: a fixed annual natural mortality of 0.52 has
been applied to individual cohorts. While this is
the only published figure for this species
(Gwyther & McShane,1988), it is likely to be
highly variable, This figure is used as an average
value only.
The scenarios modelled under the assumptions
above (Table 1) have the same population (a)
fished after initial discovery as a combination of
1-, 2- and 3+ year old cohorts, (b) left for one year
and fished at 2-, 3- and 44 year old соп», and
(c) left for two years and fished as 3-, 4- and 5+
ycar old cohorts. In this simple example substan-
tial decreases in yield result from leaving the
scallops until a majority are aged 2+ years and
older, and this trend is continued if they are not
fished until the dominant cohort is 3 years old.
However, itis worth considering this result more
closely in relation to actual fishing practice in
Bass Strait. Scallops in year 1 cohorts in July of
each year typically range in size from 50-70mm
shell height, but it is unlikely that many would
either be landed or processed that were smaller
than about 60mm shell height (ignoring the legal
minimum size limit of 70mm), thus explaining
the necessity for calculation of some effective
exploitation rate, E, Assuming a normal size dis-
tribution of scallops in the cohort, only one half
of the cohort would then be converted to ‘yield’,
This has been calculated in Table 1 (d), where it
is clear that yield from this bed is substantially
reduced if this strategy is used, rather than leaving
the bed until the major cohort was 3 years old.
This size selectivity can normally be accounted
for, and adjusted at will in yield models, bul is
kept separate here for simplicity.
Regardless of the yield implications, the impor-
tant result for a management strategy utilizing
scenario (b) and (c) 1s in potential egg production
as a measure of recruitment overfishing. Using
(Fig. 3):
Fecundity (millions of eggs) = 1.086 (years
old) + 0.148 (5).
it is a simple matter to calculate the difference
in age-based egg production from the three
management strategies (Table 1). The differences
TABLE 2. Egg abet for three management
strategies, based on analytical yield model of Table 1
and age/fecundity schedule of Figure 3; where (a) is
scallops fished from age 1+ onwards, (b)i is scallops
fished from age 2+onwards, and (c) is scallops fished
from age 3+ onwards
Pop. size size ET Cumulative -
(000.000) ers ерр production
|
(a)
[3| — 310 |
[ | тота ээ | 359 |
&|2|] 22 | 283 | |
283
à os | 25 | |
[4| a 1 —— — |
L [reme a | юг /
Si ee БУА
6
Бү -0.14B62-- |.0862« А^2 = 0.528
4
Fecundity (miilions o! eggs per female)
2
1
D
0 \ 2 3 &
Age (years)
FIG. 3. Age (years) versus fecundity for six Bass Strail
scallop beds.
in egg production fram these strategies are evi-
dent from Table 2, where it is obvious thal while
most eggs are released when population size is at
its highest, removing the population al this point
will result in only a fraction of the total egg
production that would have been realised if the
population had been left until the major cohort
was 3 years old, that js, the population spawns in
each year of the three years until caught.
The difference in moving from management
strategy (d) to strategy (c) will result in only a
20% drop in total yield, but a 260% increase in
egg production. Over the medium to long term it
1s probable that this will be reflected in the
recruitment strength of scallop stocks, as well as
in higheroverall prices from larger sized scallops.
DISCUSSION
Given the high per-unit value and the relative
ease of catching scallops when they are abundant,
it is hardly surprising that continued consumer
demand has had a significant impact on invest-
ment and effort in the Bass Strait fishery. Other
complicating factors include ‘diversified’ fishing
license policies in Tasmania, which maintains a
large pool of potential effort, and conversely, the
lack of diversified license policies in Victoria
which forces effort into the fishery because of the
inahility of the licence holders to spread effort to
other fish stocks, There js no easy management
solution here, but is becoming evident that there
is a need for bioeconomic modeling to determine
not only effects of ecological and environmental
variables on scallop stocks, hut also the critical
MEMOIRS OF THE QUEENSLAND MUSEUM
problems of social and economic pressures on the
scallop fishing industry and how these relate Lo
stack managernent (Caddy, 1989),
A structural problem in the Bass Strait fishery
is that these same social and economic pressures,
and a lack of management foresight in the late
1970's and early 1980's, have resulted in a Песі
capable of annual overfishing of available stocks.
The management plan introduced in 199] con-
tains a mechanism for avoidance of the most
serious long term problem, recruitment overfish-
ing. The two spawnings criterion goes a long way
to solving this chronic overfishing problem while
also attempting to maximise the yield from exist-
ing recruits, but the economics of fishing will
remain marginal while stocks remain low. This
linking of ‘yicld-per-recruit’ with 'eggs-per-
recruit’ is a valuable extension of the analytical
yield model and was used by Mohn et al. (1984)
to develop management strategies for Georges
Bank scallops (Gabriel et aj.,1989). The
mechanism for achieving the two spawnings
policy, that is, a trashing rate tied to a size limit,
does not reduce its medium and long term validity
and is also a valuable management tool for con-
servation of juvenile stocks. The trashing rate
concept will also become increasingly important
as (hopefully) stocks rebuild and new juvenile
beds are discovered.
Similar policies for shifting fishing effort from
younger age groups to older and larger age groups
have resulted in substantial gains in long term
yields and egg production in Canadian scallop
fisheries (Caddy,1989). Targeting of older agc
group scallops had a number of beneficial im-
pacts on fishing strategies not characteristic of
simple minimum size limits, which has been sug-
gested as an alternative policy for the Bass Strait
fishery. A minimum size limit without a viable
system of protecting dense patches of new
recruits by local area closures (particularly in
multiple year class beds) would return the fishery
to the destructive practice of fishing areas of
predominately small shell in order to cull out the
few large scallops, while inflicting high levels of
incidental mortality on unlanded juveniles (Mc-
Loughlin et al,,1988; Caddy, 1989), The fact that
this practice does occur at low stock levels in Bass
Strait is evident from examination of the 1987
season (Martin et al,,1989),
The principle of the "two spawnings' criteria is
nothing more or less than one strategy for stock
rebuilding. However, a stock rebuilding strategy.
with the specific objective of increasing spawn-
ing stock abundance, is just one example of a
MANAGEMENT OF BASS STRAIT SCALLOP FISHERY
fishery management strategy (Sainsbury, 19922).
The management strategy may include the use of
biological or fishery reference points and a
specified way in which the reference points are to
һе used in the management of the fishery (e.g. no
fishing on stocks until they have completed two
spawnings), However, as Sainsbury (19922) ex-
plained, the essential point is that the manage-
ment strategy to be evaluated is but one aspect of
a process that includes stock dynamics, economic
dynamics, observations (of fleet and fishery per-
fotmance), estimation procedures, management
decisions and management implementation, all
operating under a management policy with
specific goals or objectives.
The exact causes and mechanisms of recruit-
ment collapse are poorly known, although often
à combination of high fishing mortality and en-
vironmental variability is indicated, and ecologi-
cal interactions are suspected, The complexity of
these interactions is such that there is little expec-
tation that the population size at which recniit-
ment collapse will happen can be accurately
predicted (Sainsbury,1992b). However, recruit-
ment collapse has occurred in numerous marine
resources, and it is strongly suspected in Bass
Strait scallops from 1986-1990.
Ultimately, the reliability and success of any
management strategy (for Bass Strait scallops) is
seen to be dependent ой the ability to forecast
accurately (Mohn,1986), although this may re-
quire a long time series of catch data to be reliable
(Orensanz et al.,1991). However, with large in-
terannual fluctuations in recruitment, growth and
mortality typically occurring in Bass Strait scal-
lops such a predictive capability is not yet pos-
sible. Management must therefore rely on
maximising probability for both maximal annual
yield and recruitment success in subsequent
years, and these strategies musi be maintained
over a suitable time period to determine if they
are successful.
In respect of Bass Strait scallops, the current
management strategy is a stock rebuilding
strategy based on output controls: two spawnings
bused on an average size al age, and catch restric-
tions, unrelated to stock size but implemented for
orderly marketing and processing. However,
there remains the risk that the underlying struc-
tural problems remain unattended. These are,
critically, (1) overall fleet size, and (2) a lack of
knowledge of stock size, resilience and produc-
tivity, The lack of а link between annual total
catch and stock size is particularly worrisome as
none of the usual stock management strategies
313
linking catch and stock, such as proportional es-
capement, constant escapement or proportional
harvesting rate can be, or arc, being applied. What
is being applied is a constant quota which is
generally recognised as a high risk strategy since
itignores interannual recruitment variability. For
example, one possible scenario is that even with
à two spawnings strategy, threc adverse ycars of
environmental conditions for recruitment will see
all stocks (beds) available for fishing, and with
the existing excess fishing capacity and with an-
nual natural mortality, stock collapse would be
once again a real possibility. Further, the existing
management plan theoretically allows ap-
proximately 18,000 tonnes of scallops (live
weight) to be. landed in any year {ie,, No, of
vessels in fleet x monthly quota x no, of months
available for fishing), despite the knowledge that
in the history of the fishery no more than 12,000
tonnes has been landed in any one year.
What then might be à course of action for
considering these problems ? Initially, an assess-
ment of the existing policy with regards to stock
recovery will be necessary. Assuming that the
stock does recover to some level consistent with
a more constant level of annual recruitment, then
in the medium term a linking of annual catch with
stock size will become necessary for some level
of sustainability (that is, an estimate of minimum
spawning stock biomass-per-recruit). Of course,
thisis inextricably linked with profitability for the
fleet, with the economics of fishing for the exist-
ing fleet only being Viable at relatively high stock
sizes - financial viability at lower stock sizes will
rely on the reduced catch being shared among
fewer operators. A policy objective in the
medium term may well be an appropriate reduc-
tion in fleet size to a level at which economic
viability is maintained at average annual catches.
ACKNOWLEDGEMENTS
This paper has grown out of discussions with
many people, including people at all levels in the
fishery - fishermen, managers and scientists, Par-
ticular thanks to AFMA, industry and state
government members of the Bass Strait scallop
Consultative Commillee, Mr Will Zacharin and
Drs. Peter Young, Tony Smith and Tony Koslow.
Mr Richard Manin produced the biological
calendar for scallops, and I thank him for permis-
sion to use itin this paper,
E
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Workshop, Hobart'. (Tasmanian Government
Printer: Hobart).
THE IMPACT OF SCALLOP DREDGING ON A SOFT SEDIMENT COMMUNITY
USING MULTIVARIATE TECHNIQUES
D.R. CURRIE AND G.D. PARRY
Currie, D.R. & Parry, G.D. 1994 08 10: The impact of scallop dredging on a soft sediment
community using multivariate technigues. Memoirs of rhe Queensland Museum 36(2): 315—
326, Brisbane, ISSN 0079-8835,
Changes to benthic infauna caused by scallop dredging in Port Phillip Bay were examined
experimentally using à BACI (Before, Alter, Control, Impact) design. Analysis of 150x0.1
т? grab samples obtained from 2 pre-dredging and 3 post-dredging periods are described.
A diverse fauna of 204 invertebrate species and 49,044 individuals were surveyed. Bray-
Curtis community dissimilarities were used to assess changes to community structure
following dredging. Pair-wise comparisons of community dissimilarity between the contral
and dredge plots through time enabled a test of the statistical significance of change following
dredging. Multi-dimensional scaling (MDS) was used to describe patterns of change follow-
ing dredging.Statistically significant (0.05«p«0.10) changes to community structure were
Ded following dredging; ecologica] significance of these changes requires Further
analysis.
Р.К, Currie and С, D. Parry, Victorian Fisheries Research Institute, P.O. Bax 114,
Queenscliff 3225, Vietoria; 15 April, 1994.
The scallop industry in Port Phillip Bay is one
of the most valuable commercial fisheries in Vic-
toria and since its establishment in 1963 has
produced up to 2000 tonnes, worth c.$20 million,
annually. Scallop dredging in Port Phillip Bay is
also widely regarded in the Victorian community
аз environmentally damaging. Many changes to
the ecology of Port Phillip Bay, noted by fisher-
men and others, have been attributed (rightly or
wrongly)to scallop dredging. In response to these
concerns, a series of linked physical (Black &
Parry, this memoir) and biological studies were
initiated in 1991 to provide information on the
impacts of scallop dredging.
Shellfish dredging may cause a range of im-
pacts (Messieh et al., 1991, Jones, 1992), bui few
are well-documented and biological impacts are
particularly difficult to investigate because of the
complexity of benthic communities and our
limited knowledge of its natural variability (Mes-
sich ct al.,1991). Early studies (Caddy,1973,
Butcher et al,,1981) of the effect of dredging on
benthic communities were qualitatitive. More
recent quantitative studies involve experimental
manipulations, but often lack the statistical power
to detect a small impact (Petersen et al,,1987,
McShane,198], Eleftheriou & Robertson, 1992)
or involve an inappropriate scale of impact, i.e.
the experimentally dredged site is much smaller
than would be dredged during normal commer-
cial activities (MeShane, 1981, Eleftheriou &
Robertson, 1992). Furthermore, the impacts of
scallop dredging depend upon the type of gear,
amount of ground contact, type of seabed, depth,
and strengths of currents (Jones,1992). The ex-
tentof biological impacts must also depend on the
vulnerability of the benthic communities.
Mostof the world's scallop dredge fisheries use
different gear, operate on a range of substrate
types and harvest scallops from different biologi-
cal communities. Consequently, even if the ef-
fects of scallop dredging had been investigated in
several of the world's fisheries, it would not be
surprising if the impacts differed.
The species most likely to be impacted by scal-
lop dredging are those which live near scallops,
on or just beneath the sediment surface, and
which are not mobile enough to avoid dredges,
Thus epifaunal and infaunal communities appear
to be the most vulnerable to scallop dredging,
This paper examines the effect of scallop dredg-
ing on infaunal communities.
Dredge-related changes to the abundance and
diversity of infaunul animals were examined
using a BACI (Before After Control Impact)
design (Stewart-Oaten et al,,1986), This design
involves simultaneous sampling of two plots (опе
control, and one dredge) on a number of oc-
casions, both before and after experimentally
dredging the ‘dredge’ plot. On each sampling
occasion differences between plots were assessed
using the Bray-Curtis dissimilarity measure and
a t-test was used to determine whether changes to
this dissimilarity measure following dredging
were statistically significant.
Changes to commumty structure following
316
N . [em Melboume
| y. à
[| \
-
„7 `
+ NL
Geelong — —/ Port Phillip Bay \
Ѓ 3 7 = Portarlington )
Ne \ a S Leonards
E ^ Dremana a^ |
—' BassStrait ^. ——— РЕ:
5 0 5 10 [5 Ww › 43
M M7
FIG.1, Map of Port Phillip Bay showing locations of
main study areas used For scallop dredging trials.
dredging were also determined using multi-
dimensional scaling (MDS). MDS provides a
means of reducing large and complex data sets so
that ecologically meaningful patterns and trends
are more apparent and more readily interpreted
(Gamito & Raffaelli,1992), MDS is a powerful
ordination procedure that attempts to place some
measure of similarity between objects into 2 or
more dimensional space, such that distances be-
tween objects correspond closely to the input
similarities. While the computational algorithm
for MDS is complex the graphical representation
is conceptually simple and easily communicated
(Clarke,1993).
METHODS
Srupv DESIGN
This study is part of a much larger study ex-
amining dredging-related changes to the abun-
dance of benthic animals in 3 areas of Port Phillip
Bay (St Leonards, Dromana and Portarlington)
during, 1991 (Parry & Currie, 1992), We describe
only studies in an area near St Leonards closed to
all scallop dredging during 1991 (Fig.1).
Two adjacent &Q0mx600m experimental plots
were located in I2-15m of water, c.2km off-
shore from St. Leonards. The more southerly was
experimentally dredged by commercial vessels
(dredge plot) and the other plot was left un-
dredged ('control' plot).
The *dredge' plot was commercially dredged
over 3 days (16-18 July, 1991) by a fleet of 4
scallop vessels, using 3m wide *Peninsula’
dredges fitted with scraper/cutter bars that did not
extend below the level of the skids (Hughes,
MEMOIRS OF THE QUEENSLAND MUSEUM
1973), Dredging was conducted for a maximum
of 3 hours per day and coincided with periods in
which there was a strong southerly tidal current
that carried any dredging-related sediment away
from the adjacent control site. The experimental
plot was dredged with a moderately high fishing
intensity compared to historical levels of fishing
in Port Phillip Bay (Parry & Currie, 1992), A 2x
dredging intensity (where 2x refers to the number
of times a dredge would on average pass over any
point within the plot) was chosen as this level of
fishing was common in areas with high densities
of scallops and because any lower intensity
would have left too large a proportion of the
‘dredged’ plot undredged.
On the first morning of the experimental dredg-
ing the plot to be dredged was marked out with 4
equidistant large buoys along each side of the
600m x 600m plot using a Furuno GP 500 GPS
Navigator connected to a colour video plotter.
This GPS provides an accuracy of 15-25m in
95% of fixes, Where inaccuracy exceeded 25m
due to intentional degradation of the system
(selective availability) this was obvious on the
plotter. The buoys marked out three 200m x 600m
lane Ways directed E-W. Scallop vessels dredged
these lane ways sequentially and fishermen were
encouraged to dredge the whole area as evenly as
possible. On the second and third days of dredg-
ing the buoys marking out the lanc way boun-
daries were moved 50m N and S of their initial
positions to minimise any undredged "shadows'
resulting from vessels not dredging near the
buoys.
Estimates of the distribution and abundance of
animals living within the sediments at cach plot
were determined from replicate 0.1 m? Smith-
McIntyre grab samples, 15 samples were taken
from each plot on 2 sampling dates before
(13/5/91, 02/7/91) and 3 after (18/7/91, 9/8/91] &
31/10/91) the experimental dredging. Each plot
was sub-divided into 12 equal sectors to facilitate
stratified random sampling; one grab was taken
at random from within each sector and the
remaining 3 grab samples were taken at random
across the plot. Samples were drained, weighed
and à 70ml subsample retained for sediment
analysis. All animals retained on a Imm sieve
were sorted to an optimal taxonomic level
(generally species) under a dissecting micro-
scope, before being counted.
DATA ANALYSIS
Differences between the control and dredge
plots at each sampling period were examined
IMPACT OF SCALLOP DREDGING ON SOFT BOTTOM COMMUNITY
CONTROL
pheta/501 124
(DOIN
teda]
thet BT
Se An 0/o1
FIG.2. Schematic diagram showing differences be-
tween the number of species and number of shared
species between different plots and sampling dates.
Numbers in large squares are total number of species
found on the control and dredge plot on each sampling
date (11—15). Black squares show the number of
species on the dredge site following the experimental
dredging. Other numbers are the number of species
shared between different plots and sampling times.
using Bray-Curtis (B-C) dissimilarity measures
(Bray & Curtis, 1957).
The Bray Curtis dissimilarity measure is:
Y | Yj-Yu |
du LE
=
PNIS
i=l
where Уу = the score for the ith species im the
jth sample; Yi, = the score for the ith species in
the kth sample; бу = dissimilarity between the
jih and kth samples summed over all s species.
This particular measure was chosen because 1) it
is not affected by joint absences 2) it gives more
weighting to abundant species than rare ones, and
3) it has consistently performed well in preserv-
ing ‘ecological distance’ in a variety of simula-
tions on different types of data (Faith et al., 1987).
On each sampling date the number of in-
dividuals of each species was calculated from the
total number of individuals found on each plot.
i.c, data from the 15 replicate grabs on each plot
were pooled. Before calculating the B-C dis-
similarity measures a double square root transfor-
317
mation was applied to the number of individuals
of each species. This transformation prevents the
abundant species from influencing the B-C dis-
similarity excessively.
Five pairwise B-C dissimilarity measures com-
prising all contro! plot versus dredge plot com-
parisons for the 5 sampling periods (2 before and
3 after dredging) were used in the BACI analysis
as proposed by Faith et al. (1991). The null
hypothesis of no dredging effect is rejected if the
mean of the B-C dissimilarity measures before
dredging is lower than that after dredging, as
judged by а 1 test.
Bray-Curtis dissimilarity measures calculated
for all 10 plot*date (2 plots x 5 dates) combina-
tions, resulted in a triangular matrix of dis-
similarities which were used to map the plot*date
inter-relationships in two dimensions. Hybrid
multidimensional scaling (Belbin,1990) was
employed for the ordination. This technique is а
hybrid between metric and non-metric multi-
dimensional scaling that attempts to combine the
best features of each of the two techniques (Faith
et 3l.,1987). By specifying a ‘cut-value’ less than
the lowest dissimilarity measure, monotonic
regression was used. The final configuration
presented is the best solution (i.e. it exhibited the
lowest ‘stress’ value = least distortion) from 100
random starts.
RESULTS
204 invertebrate species and 49.044 individuals
were encountered at the 2 St Leonards plots
during the course of this study (Appendix); 86
(42%) were crustaceans, 53 (26%) polychaetes,
38 (19%) molluscs, and 27 (13%) members of
other phyla.
At St Leonards, as is common with most other
ecological communities (Preston, 1948), there are
а small number of abundant species and a large
number of relatively rare species. The amphipod
Photis sp.1 was the most abundant species and
contributed 35% of the animals collected. Collec-
tively the 20 most abundant species contributed
85% of the animals collected. By contrast, 105
species were represented in fewer than 10 of the
150 grab samples taken, and 38 species occurred
in only one grab.
CHANGES IN SPECIES NUMBERS
The difference beween the total number of
species sampled on the control and dredge plots
was small before the dredging (5 att1, | at t2; Figs
2,3) but increased following dredging (8 at 13, 3]
318
150
125
100
No. of Species
a
150
200
Time (days)
FIG.3. Total number of species recorded in 15 replicate
grab samples taken from the control ([ ]) and dredge
(Bl) plots. The broken line indicates the number of
species shared between the two plots. Arrow indicates
when experimental dredging occurred.
at t4, 15 at t5; Figs 2,3). The number of species
shared between the control and dredge plots
decreased from 101(t1) and 97(t2) before dredg-
ing to 85(t3), 82(t4) and 93(t5) following dredg-
ing (Figs 2,3). Other comparisons of the number
of species shared between sampling times (Figs
2,3) also suggest that there was a reduction in the
number of species following dredging. Over all 5
sampling times 72 species were always found on
the control plot, but only 62 were always found
on the dredge plot.
The mean difference in species number be-
tween both plots increased from 3 before dredg-
ing to 18 after dredging. A t-test of this increase
in difference after dredging was significant at
10000
15
7500
8
© t4
© 50004"!
c t2 t3
a
a
2500 5
0
0 50 100 150 200
Time (days)
FIG.4. Total number of individuals in 15 replicate grab
samples taken from the control ([ ]) and dredge (Bl)
plots. Arrow indicates when experimental dredging
occurred.
MEMOIRS OF THE QUEENSLAND MUSEUM
0.30
0.25
0.201!
045 !
0.10
Dissimilarity
0.05
0.00
0 50 100
Time (days)
150 200
FIG.5. Bray-Curtis community dissimilarity between
control and dredge plots before and afterexperimental
dredging. Arrow indicates when experimental dredg-
ing occurred.
0.05«p«0.10. However the power of this test to
detect a change of the observed magnitude was P
«0.30 when а=0.05.
CHANGES IN NUMBERS OF INDIVIDUALS
The total number of individuals of all species
sampled on the control plot and the dredge plot
increased between tl and t5, and particularly
between (4 and t5 (Fig.4). This increase is the
result of recruitment of juveniles, particularly of
Photis sp.1, which accounts for approximately
half of the overall increase during the study
period (Currie & Parry, unpubl. data). However
at each sampling time following dredging (13—15)
the number of individuals on the dredge plot was
0.30
0,25
0.20
0,15
Dissimilarity
100
Time (days)
0 50 150
FIG.6. Bray-Curtis community dissimilarities between
successive sampling dates (11—12, 12—13, 12—14, t4-15.
at control ([ ]) and dredge (MM) plots. Broken lines
indicate t1-t5 comparisons for the control. ([]) and
dredge (Bl) plots. 11 = 0 days.
IMPACT OF SCALLOP DREDGING ON SOFT BOTTOM COMMUNITY
lower than the number on the control plot,
whereas before dredging there were either similar
numbers on both plots (12) ar more on the dredge
plot (t1, Fig.4).
Community DISSIMILARITY
Bray-Curtis dissimilarity measures between
the control and dredge plots on the 5 sampling
dates (Fig.5) increased significantly (t-test,
0.05<p<0.10) from a mean of 0.175 before dredg-
ing to 0.211 after dredging, but the power of this
test to detect a change of the observed magnitude
was low (Р<0.32 when @=0.05), The first post-
dredging sampling (t3) occurred on the last day
of the experimental dredging and at this time
there was minimal change in community dis-
similarity, but the dissimilarity between the plots
increased after 23 days (t4) before decreasing
again after 88 days (15). The increase in dis-
similarity between t3 and t4 may have resulted
from some moribund animals being collected on
the dredge plot at 13, but these would not have
been distinguishable from healthy animals in our
analysis. Alternatively dredging may cause in-
direct ecological changes, such as increased vul-
nerability to predation, which take some time to
have their maximum impact. The apparent in-
crease in similarity of the plots between t4 and t5
is probably ihe result of recruitment of many
additional species on both plots during this
period, Recruitment of Photis sp.l at this time
makes only a small contribution to the B-C dis-
similarity as a similar pattern of dissimilarity
measures was obtained using only species
presence-absence data (Currie & Parry, unpubl.
data).
Comparison of Bray-Curtis dissimilarities be-
tween successive dates on the dredge and control
plots (Fig.6) demonstrate that before dredging
(11—12) there was little difference between succes-
sive samples. On the contro] plot following
dredging there is a decrease in community dis-
similarity in the periods 12—13 and t3-t4, whereas
on the dredge plot community dissimilarity in-
creases in these same periods. On both the control
and dredge plots there is an increase in dis-
similarity in the period 14—15 apparently due to
recruitment of animals (particularly additional
species) to both plots. Over the entire study
period t1-t5 there was a larger increase in dis-
similarity on the control plot than on the dredge
plot. This appears to be thc result of relatively
lower recruitment of additional species on the
dredge plot than on the control plot in the period
319
BR =—=ъ
Axis 1
FIG.7, Two-dimensional scaling ordination mapping
the relationships between benthic communities on the
control (C) and dredge (D) sites before and after
dredging, Numerals indicate the date of sampling (i.e.
1213/5/91; 2=2/7/91, 3=18/7/91; 4=9/8/91;
5231/10/91. Experimental dredging was conducted
on 16, 17 and 18 July,1991, The solid lines connect
control and dredge plots sampled on the same date.
The broken line connects the different sampling times
in sequence from t1 to 15,
following dredging, and suggests that dredging
may reduce larval settlement.
MULTIDIMENSIONAL ScALING (MDS)
The MDS ordination (Fig.7) maps the spatial
and temporal changes in benthic community
structure on the contro] and dredge plots before
and after dredging. The stress coefficient of
0.153, indicates that the ordination ts not unduly
distorted (Clarke, 1993), and a fair representation
of the input dissimilarities in 2-dimensions.
The MDS ordination summarises many of the
changes on the contro] and dredge plots noted
above. Length of the lines in Fig.7 provide a
measure of the dissimilarity of the dredge and
control plots through time. Short lines connectthe
control and dredge plots at the first and second
sampling dates (C1-D] and C2-D2), but imme-
diately following dredging the length of the lines
increase, indicating an increase in dissimilarity
between the control and dredge plots. The line
connecting C4-D4 is the longest which indicates
that on the second sampling date after dredging
(14) the plots are at their most different. The
subsequent decrease тп the length of the line at t5
(C5-D5) indicates that the plots are becoming
more similar.
The broken line in Fig. 7 suggests that both the
control and dredge plots follow a similar tem-
poral trajectory which probably represents
seasonal changes on both plots, The greatest tem-
poral change occurs between t4 and 15, and coin-
cides with the high levels of recruitment observed
on both plots. Consideration of changes on the
contro] plot also suggest that remporal changes
are small between t1 and t4 (C1, C2, C3 and C4
grip together) but are greater between 14 and t5
(C5 is distant fram C1, C2, СЗ and C4), The three
samples taken on the dredge plot following
dredging (D3, D4, DS) are the most divergent.
DISCUSSION
A statistically significant (0.05<p<0.10) in-
crease in the Bray-Curtis dissimilanty between
the control and dredge plots occurs following the
experimental dredging. This increase indicates
thal scallop dredging changes the benthic com-
munity structure at St Leonards. This change in
community structure appears to be the result of a
decrease in species number (Figs 2,3) and a
decrease in abundance of particular species
(Fig.4).
No previous studies have demonstrated a sig-
nificant impact of shellfish dredging on benthic
infauna, partly at least due to the low statistical
poet of the tests involved (McShane,1981,
tersen eL al., 1987). Low power results from the
large spatial variahility of benthic communities,
the apparently small changes to the abundance of
most species caused by dredging and from low
intensity of sampling. The number of benthic
os already analysed in this study far ex-
ceeds the numbers analysed in previous studies,
but still further pre-dredging and post-dredging
samples must be analysed to confirm that our
analysis 15 statistically robust. The usual statisti-
cal convention of p<0.05 has been relaxed in this
study in an effort to more nearly balance type I
and type II errors (Peterman,1990). Analysis af
the effects of dredging on individual species is in
progress and should enable identification of any
characteristics of these species that may cause
them to be vulnerable to dredging. This will
greatly reduce the risk that the changes observed
are due to an impact coincident with dredging
(‘demonic intrusion", Hulbert, 1987), às will anal-
ysis of data collected at our othertwa study sites.
Assessment of the ecological significance of
changes to community structure caused by dredg-
ing also remains to be determined. This assess-
ment requires better estimates of the percentage
MEMOIRS OP THE QUEENSLAND MUSEUM
change in abundance of various species, the per-
sistence of these changes, and information òn the
trophic and other ecological consequences of the
changes to the infauna. Studies in progress will
provide this additional information and clarify
the ecological importance of changes to benthic
communities caused by scallap dredging,
ACKNOWLEDGEMENTS
We thank the technical staff at the Victorian
Fisheries Research Institute, Queenscliff, for ex-
pert assistance. In particular we thank members
of the Scallop Dredge Effects Program for their
many hours of dedicated laboratory analysis; A.
Bury, A, Jahnecke, R, Flint, M. Forsyth, S. Frlan
& M. Miller. We would also like to thank the
skipper D. Beyer, and crew R. Metcalf and M.
Hoskins of the R.V, Sarda. Finally we thank the
Victorian Scallop Industry for their co-operation
in conducting the experimental dredging.
LITERATURE CITED
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BUTCHER, T., MATTHEWS, J., GLAISTER, J. &
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CADDY, JF. 1973. Underwater observations on tracks
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CLARKE, K.R. 1993, Non-parametric multivariate
analyses of changes in communily structure,
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ELEFTHERIOU, A. & ROBERTSON, M.R. 1992. The
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IMPACT OF SCALLOP DREDGING ON SOFT BOTTOM COMMUNITY
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Appendix
321
PARRY, G.D. & CURRIE, D.R. 1992. Interim report
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Classification of the 204 benthic invertebrate species identified from 150 Smith-McIntyre grab
samples taken at 'control' and 'dredge' plots off St. Leonards (38°10.06’S, 144? 44.80" E. between the
13 May,1991 and 31 October, 1991. Overall species rankings are given in ascending order of summed
abundances. OBS - number of grab samples in which a species occurred.
322
CRUSTACEA
AMPHIPODA:
FAM: AMPELISCIDAE.
FAM: CAPRELLIDAE.
FAM: COROPHIIDAE.
FAM: CYPROIDEIDAE.
FAM: DEXAMINIDAE.
FAM: GAMMARIDAE.
FAM: LEUCOTHOIDAE.
FAM: LILJEBORGIIDAE.
FAM: LYSIANASSIDAE.
FAM: MELPHIDIPPIDAE,
FAM: OEDICEROTIDAE,
FAM: PHOXOCEPHALIDAE.
FAM: PODOCERIDAE.
ISOPODA:
FAM: ANTHURIDAE.
FAM: ASTACILLIDAE.
FAM: EURYDICIDAE.
FAM: PARANTHURIDAE,
FAM: SEROLIDAE.
FAM: SPHAEROMIDAE.
CUMACEA:
FAM: BODOTRIIDAE.
FAM: DIASTYLIDAE.
FAM: LEUCONIDAE.
MEMOIRS OF THE QUEENSLAND MUSEUM
SPECIES.
Byblis mildura Lowry & Poore, 1985
Ampelisca euroa Lowry & Poore, 1985
Metaprotella cf. haswelliana Haswell, 1884
Photis sp.1
Ericanthonius sp.1
Aora mortoni (Haswell, 1879)
Narapheonoides mullaya Barnard, 1972
Paradexamine lanacoura Barnard, 1972
Melita sp.1
Maera mastersi (Haswell)
Ceradocus serratus (Bate)
Leucothoe assimilis Barnard, 1974
Leucothoe sp.1
Paraleucothoe novaehollandiae Stebbing, 1899
Liljeborgia sp.1
Liljeborgia sp.2
Endevoura mirabilis Chilton, 1921
Hippomedon denticulatus (Bate)
Amaryllis macrophthalmus Haswell, 1879
Lysianassid sp.1
Lysianassid sp.2
Lysianassid sp.3
Lysianassid sp.4
Cheirocratus bassi (Stebbing)
Oedicerotid sp.1
Oedicerotid sp.2
Birubius babaneekus Barnard & Drummond, 1978
Phoxocephalus kukathus Bamard & Drummond, 1978
Brolgus tattersalli (Bamard)
Birubius panamunus Bamard & Drummond, 1976
Birubius cartoo Barnard & Drummond, 1978
Dulichia sp.1
Amakusanthura pimelia Poore & Lew Ton, 1985
Haliophasma cribense Poore, 1975
Haliophasma canale Poore, 1975
Neastacilla deducta (Hale)
Natatolana woodjonesi (Hale)
Natatolana corpulenta (Hale)
Bullowanthura pambula Poore, 1978
Leptanthura diemenensis (Haswell, 1884)
Heteroserolis australiensis (Beddard)
Exosphaeroma sp. 1
Glyphocuma bakeri (Hale)
Gynodiastylis ambigua Hale, 1946
Dimorphostylis cottoni Hale, 1936
Dicoides fletti Hale, 1946
Hemileucon levis Hale, 1945
RANK
OBS.
IMPACT OF SCALLOP DREDGING ON SOFT BOTTOM COMMUNITY
Crustacea cont.
DECAPODA:
FAM: ALPHEIDAE.
FAM: CALLIANASSIDAE.
FAM: CRANGONIDAE.
FAM: DISCIADIDAE.
FAM: GALATHEIDAE.
FAM: GONEPLACIDAE.
FAM: HIPPOLYTIDAE.
FAM: HYMENOSOMATIDAE.
FAM: LEUCOSIIDAE.
FAM: MAJIDAE.
FAM: PASIPHAEIDAE.
FAM: PINNOTHERIDAE.
FAM: PORCELLANIDAE.
FAM: PORTUNIDAE.
FAM: SERGESTIDAE.
FAM: XANTHIDAE,
MYSIDACEA:
FAM: MYSIDAE.
SF: GASTROSACCINAE.
SF: SERIELLINAE.
SF: MYSINAE.
TANAIDACEA:
FAM: APSEUDIDAE.
FAM: KALLIAPSEUDIDAE.
FAM; TANAIDAE.
OSTRACODA:
S/O: CYPRIDINIFORMES.
FAM: CYPRIDINIDAE.
S/O: CYLINDROLEBERIDIDAE.
S/O: CYLINDROLEBERIDIDAE.
FAM: SARSIELIIDAE.
FAM: PHILOMEDIDAE.
COPEPODA:
ORDER: CALANOID.
ORDER: CYCLOPOIDA.
NEBALIACEA:
FAM: NEBALIIDAE.
LARVAE:
SPECIES.
Alpheus euphrosyne (de Man)
Athanopsis sp.1
Callianassa arenosa Poore, 1975
Upogebia dromana Poore & Griffin, 1979
Pontophilus intermedius (Bate)
Discias sp.1
Galathea australiensis (Stimpson)
Munida haswelli (Henderson)
Hexapus ѕр.1
Hippolyte tenuirostris (Bate)
Halicarcinus rostratus (Haswell)
Halicarcinus ovatus (Stimpson)
Phlyxia intermedia Miers, 1886
Philyra undecimspinosa (Kinahan)
Majid sp.1
Thacanophrys spatulifer (Filho!)
Leptochela sp.1
Pinnotheres hickmani (Baker)
Polyonyx transversus (Haswell)
Nectocarcinus integrifrons (Latreille, 1825)
Leucifer sp.1
Heteropilumnus fimbriatus (Milne Edwards)
Paranchialina angusta (Sars)
Siriella vincenti (Tattersall)
Australomysis incisa (Sars)
Tenagomysis sp.1
Apseudes sp.1
Kalliapseudes sp.1
Tanaidae sp.1
Cypridinidae sp.1
Bathyleberis sp.1
Empoulsenia sp.1
Sarsiellid sp.1
Philomedid sp.1
Labidocera sp.1
Cyelopoid sp.1
Nebalia sp.1
Caridea larvae sp.1
Brachyura zoea sp.1
RANK
120
137
163
SUM
ms
BAIR NK OR De _
477
323
OBS.
U^
WARNE M Mom om
324 MEMOIRS OF THE QUEENSLAND MUSEUM
SPECIES.
ECHINODERMATA
CLASS: HOLOTHUROIDEA:
FAM: CHIRIDOTIDAE.
FAM: SYNAPTIDAE.
Trochodota allani (Joshua, 1912)
Leptosynapta dolabrifera (Stimpson, 1855)
SUBCLASS: OPHIUROIDEA:
FAM: AMPHIURIDAE.
FAM: OPHIURIDAE.
CLASS: ECHINOIDEA:
FAM: LOVENIIDAE.
CHORDATA
ASCIDIACEA:
FAM: ASCIDIIDAE.
FAM: STYELIDAE.
FAM: PYURIDAE.
NEMERTINEA
PORIFERA
PHORONIDA
PROTOZOA
FORAMINIFERA:
FAM: MILIOTIDAE.
FAM: POLYMORPHINIDAE.
ECHIURA
Amphiura elandiformis Clark, 1966
Ophiocentrus pilosus (Lyman)
Amphipholis squamata (D. Chiaje, 1828)
Ophiura kinbergi Ljungman, 1866
Echinocardium cordatum (Pennant, 1777)
Ascidia sydneyensis Stimpson, 1885
Ascidiella aspersa (Müller)
Cnemidocarpa etheridgii (Hardman)
Pyura stolonifera (Heller, 1878)
Nemertean sp.1
Nemertean sp.2
Nemertean sp.3
Nemertean sp.4
Nemertean sp.5
Nemertean sp.6
Nemertean sp.7
Nemertean sp.8
Nemertean sp.9
Demospongiae sp.1
Phoronis sp.1
Triloculina affinis d'Orbigny, 1826
Quinqueloculina sp.1
Quinqueloculina sp.2
Guttulina sp.1
Metabonellia haswelli (Johnston & Tiegs)
Anelassorhynchus porcellus (Fisher)
RANK
38
11
90
118
164
169
204
SUM
120
23
1262
OBS.
70
NNW
129
IMPACT OF SCALLOP DREDGING ON SOFT BOTTOM COMMUNITY
ANNELIDA
POLYCHAETA;
FAM: AMPHERETIDAE.
FAM: CAPITELLIDAF.
FAM: CHAETOPTERIDAE,
FAM: CIRRATULIDAE.
FAM: DORVILLEIDAE.
FAM: EUNICIDAE.
FAM: FLABELLIGERIDAE.
FAM: GLYCERIDAE.
FAM: GONIADIDAE.
FAM: HESIONIDAE.
FAM: LUMBRINERIDAE,
FAM: MAGELONIDAE.
FAM: MALDANIDAE.
FAM: NEPTHYIDAE.
FAM: NEREIDAE.
FAM: OPHELLIDAE.
FAM: ORBINIIDAE.
FAM: PARAONIDAE.
FAM: PECTINARIIDAE.
FAM: PHYLLODOCIDAE.
FAM: POLYNOIDAE.
FAM: SABELLIDAE.
FAM: SERPULIDAE.
FAM: SIGALIONIDAE,
FAM: SPIONIDAE,
FAM: SYLLIDAE,
FAM: TEREBELLIDAL.
FAM: TRICHOBRANCHIDAE,
SPECIES.
Ampharete sp.1
Capitellid sp.1
Notomastus sp.1
Notomastus sp.2
Chaetopterus variopedatus (Renier, 1804)
Chaetozone sp.1
Tharyx sp.1
Dorvillea australiensis (M'Intosh, 1885)
Marphysa sp.1
-Diplocirrus sp.1
Glycera cf. americana Leidy, 1855
Goniada emerita Audouin & Milne Edwards, 1833
Ophioglycera sp.1
Nerimyra longicirrata Knox & Cameron, 1971
Hesionid sp.2
Lumbrineris latreilli Audouin Milne Edwards, 1834
Magelona cf. dakini Jones, 1978
Clymenella sp.1
Asychis sp.1
Maldanid sp.1
Nephtys inornata Rainer & Hutchings, 1977
Simplisetia aequisetis Hutchings & Turvey, 1982
Olganereis edmondsi (Hartman)
Platynereis dumerilii antipoda Hartman, 1954
Ceratonereis sp.1
Armandia cf. intermedia Fauvel, 1902
Polyophthalmus pictus (Dujardin, 1839)
Leitoscolopolos bifurcatus (Hartman, 1957)
Aricidea sp.1
Paraonid sp.1
Paraonis gracilis gracilis (Tauber, 1879)
Pectinaria cf. antipoda Schmarda, 1861
Phyllodoce sp.1
Eulalia sp.1
Paralepidonotus ampuiliferus (Grube, 1878)
Harmothoe sp.1
Harmothoe spinosa Kinberg, 1855
Malmgrenia microscala (Kudenov)
Jasmineira ѕр.1
Myxicola infundibulum (Renier, 1804)
Serpulid sp.1
Sigalion sp.1
Prionospio coorilla Wilson, 1990
Prionospio yuriel Wilson, 1990
Polydora sp.1
Laonice quadridentata Blake & Kudenov, 1978
Syllis sp.1
Amaenna trilobata Hutchings & Glasby, 1986
Terebellid sp.1
Eupolymnia koorangia Hutchings & Glasby, 1988
Terebellides sp.1
Artacamella dibranchiata Knox & Cameron, 1971
RANK
SUM
325
OBS.
326
MOLLUSCA:
FAM: AGLAJIDAE.
FAM: ARCIDAE.
FAM: CARDIIDAE.
FAM: CORBUI IDAE.
FAM: CYAMIIDAE.
FAM: DORIDIDAE.
FAM: EULIMIDAE.
FAM: GONIODORIDIDAE.
FAM: HAMINEIDAE.
FAM: HIATELLIDAE.
FAM: KELLIIDAE.
FAM: MACTRIDAE.
FAM: MONTACUTIDAE.
FAM: MURICIDAE.
FAM: MYTILIDAE.
FAM: NASSARIDAE.
FAM: МАТІСІРАЕ.
FAM: NUCULIDAE,
FAM: OSTREIDAE.
FAM: PECTINIDAE.
FAM: PERIPLOMATIDAE.
FAM: PHILINIDAE.
FAM: PTERIIDAE
FAM: PYRAMIDELLIDAE.
FAM: SEMELIDAE.
FAM: SOLENIDAE.
FAM: TELLINIDAE.
FAM: TROCHIDAE.
FAM: VENERIDAE.
MEMOIRS OF THE QUEENSLAND MUSEUM
SPECIES.
Aglaja taronga Allan, 1933
Anadara trapezia (Deshayes, 1840)
Pratulum thetidus (Lamarck, 1819)
Fulvia tenuicostata (Lamarck, 1819)
Corbula cf. сохі Pilsbury, 1897
Cyamiomactra communis Hedley, 1905
Doris cameroni (Allan, 1947)
Strombiformis topaziaca (Hedley, 1908)
Okenia sp. nov.
Liloa brevis (Quoy & Gaimard, 1834)
Hiatella australis (Lamarck, 1818)
Hiatella subulata (Gatliff & Gabriel, 1910)
Melliteryx acupunctum (Hedley, 1902)
Mactra jacksonensis (Smith, 1885)
Mysella donaciformis Angas, 1878
Bedeva paivae (Crosse, 1864)
Amygdalum beddomi Iredale, 1924
Musculus ulmus Iredale, 1936
Nassarius (Zeuxis) pyrrhus (Menke, 1843)
Polinices sordidus (Swainson, 1821)
Sinum zonale (Quoy & Gaimard, 1833)
Nucula pusilla Angas, 1877
Nucula obliqua (Lamarck, 1819)
Ostrea angasi Sowerby, 1871
Pecten fumatus Reeve, 1852
Offadesma angasi (Crosse & Fischer, 1864)
Philine angasi (Crosse & Fischer, 1865)
Electroma georgiana (Quoy & Gaimard, 1835)
Pyrgiscus fusca (A Adams, 1853)
Theora cf. lubrica H & A Adams, 1866
Solen vaginoides (Lamarck, 1818)
Tellina (Macomona) mariae (Tenison Woods, 1875)
Ethminolia vitilignea (Menke, 1843)
Chioneryx cardioides (Lamarck, 1818)
Callanaitis disjecta (Perry, 1811)
Placamen placida (Philippi, 1835)
Venerupis sp. Lamarck, 1818
RANK. SUM.
95 17
99 16
107 14
160 2
7 1974
162 2
187 1
158 2
181 1
41 92
69 34
159 2
94 T
128 7
136 5
140 4
135 5
149 3
53 62
139 4
182 1
49 71
3 15
184 1
81 23
54 57
48 72
150 3
148 3
24 269
183 1
185 1
147 3
26 228
138 4
186 1
161 2
OBS.
кою Д л кю кю Ы
©
_
ee MAUS
SEDIMENT TRANSPORT RATES AND SEDIMENT DISTURBANCE
DUE TO SCALLOP DREDGING IN PORT PHILLIP BAY
KERRY P, BLACK AND GREGORY D, PARRY
Black, K.P. & Parry, G.D. 1994 08 10: Sediment transport rales and sediment disturbance
due to scallop dredging in Port Phillip Bay. Memoirs of the Queensland Museum 36(2):
327-341. Brisbane. ISSN 0079-8835.
The firstdirect measurements of turbidity caused by scallop dredging are presented. The
physical effects of scallop dredging on the sediment dynamics of an enclosed, heavily-fished
bay in southern Australia are indicated and data are provided to assess potential biological
impact, Transport and deposition of sediments were measured within and beyond the
sediment plume behind a scallop dredge. Natural suspended sediment concentrations were
recorded with a bottom-mounted instrumented frame; sediment disturbance behind dredges
was determined using the same instrumentation mounted on a towed sled. Concentrations in
the sediment plume 2-16 seconds after dredging were 2-3 orders of magnitude higher than
natural concentrations. Plume concentrations were similar to the natural levels after c. 9
minutes. Thus, for typical currents of approximately 0.1m.s:!, suspended concentrations
above natural levels were confined to a region within c.54m of the dredge. However, the fine
material remained 1n suspension longer, so dredging may be partially responsible for
re-distribution of fine sediments in the bay,
Kerry Black, Victorian Institute of Marine Sciences, 23 Sr Andrews Place, Melbourne,
Victoria 3002; Greg Parry, Victorian Fisheries Research Institute, PO Box 114, Queenscliff,
Victoria 3225; 15 April, 1994.
Scallop dredging is the most valuable commer-
cial fishery in Port Phillip Bay with annual har-
vests worth up to $20 million. In à typical year
dredges disturb approximately 400k m^ (2096) of
the bed of Port Phillip Bay (Parry, unpubl. data).
Thus, after a season of fishing, dredging repre-
sents a potential disturbance to sediment which
may be equivalent to natural phenomena, par-
ticularly in deeper water where bottom wave
energies are lower. By suspending the surface
layer of sediment, dredging may be responsible
for disturbance of previously buried material,
Direct disturbance of fine sediments may result
in the release of heavy metals, nutrients or toxic
algal spores. Alternatively, dredging may simply
break natural sediment bonds, allowing more
suspension 10 occur during natural storms, Grain
size and natural turbulence levels in the bay will
determine where the sediments settle again.
This study tests whether dredging alters tur-
bidity in the Bay and examines sedimentation
patterns after a dredge passes through a region.
To develop an appropriate quantitative com-
parison, both the natural and the dredge- related
sediment concentrations were recorded. We are
concerned with the physical sediment transport
processes only; turbidity in the plumes, sedimen-
tation, depth of disturbance and changes to the
natural bonds in the sediments, Other factors such
as incidental mortality of scallops and other
marine organisms, impact on habitat and short to
medium-term impacts on biological communities
were treated separately; the latter is presented
elsewhere (Currie & Parry, this memoir),
Currents can be tidal, wind-driven, forced from
Bass Strait or associated with internal density
structure (Environmental Study of Port Phillip
Bay,1973; Black,1993), Wave orbital motion
which determines sediment transport rate is a
function of water depth, wave height, wave per-
iod, wind strength and wind fetch. Grain sizes of
suspended sediments during storms vary across
the bay and decrease with distance above the bed.
Sediment concentrations in suspension are also a
function of the cohesiveness of the sediments,
and cohesiveness may be reduced after dredging.
To cater for the wide range of natural condi-
tions, measurements of natural sediment
dynamics required simultaneous time series of
forcing factors, including current strengths and
wave activity, and the grain size distribution of
bed sediments. Natural sediment concentrations
and hydrodynamic variables were measured con-
tinuously with in situ bottom-mounted instru-
ments, which were deployed for periods of
several days to several weeks at three sites in the
bay. These measurements complemented an ex-
isting numerical hydrodynamic model of the Bay
(Black et al..1993) and earlier sediment transpart
and wave studies (Black & Rosenberg.1992).
328 MEMOIRS OF THE QUEENSLAND MUSEUM
LEGEND
[9] Current meter only sites
SEDCAM and current meter sites
е
AUSTRALIA Ж, Areas closed to dredging
— Sediment trap transect
RIVER
YARRA
MELBOURNE
0 5 10 15 km
Depths in metres „ Laverton
G/6368
Pt. Cook
i
ъ
a
ORT PHILLIP BAY
Portarlington
GEELONG St Leonards e ^
Ce,
Ex,
SEW
No * Dromana
BASS STRAIT D
7
FIG.1. Study region where instruments were deployed. The experimental plots аге shown at Dromana, Portar-
lington and St Leonards.
SEDIMENT TRANSPORT AND DISTURBANCE PORT PHILLIP BAY
ROMANA
PERCENTAGE
FECRIARI INGTON
РЕН ЕН TAGE
ST LEDNARES
PERGENIAGL
GRAIN Size (PHI)
FIG.2, Representative bed sediment grain sizes at 3
study sites.
Measurements within the sediment plume of à
scallop dredge required development of a towed
monitoring sled. The sled was instrumented with
turbidity monitors and pumps, the latter to sample
for grain size and to calibrate turbidity monitors.
The overall study forms part of a series of
linked biological and physical investigations,
commenced in 1991 in response to a Victorian
Government initiative (Parry & Currie,1992;
Currie & Parry, this memoir). The objectives
were to: 1) compare quantitics of sediment put
into suspension by scallop dredging with natural
sediment transport due to storms; 2) develop a
sled-mounted monitoring system to be towed be-
hind a scallop dredge to determine the amount of
sediment disturbed by dredges of different
design, the influence of vessel speed, cable length
and sediment type on the amount of sediment
329
disturbed and catch efficiency of dredges of dif-
ferent designs, the optimal compromise between
the catch efficiency and sediment disturbance:
and 3) determine sedimentation patterns and
areas of influence arising as 4 consequence of
suspension of sediments by scallop dredging.
In this paper, we describe the scope of the
overall study, and present results associated with
the first objective and aspects of the second.
STUDY REGION
Port Phillip Bay (PPB) is a large, semi-
enclosed, predominantly tidal embayment linked
to the ocean by a narrow, rocky entrance (Fig. 1).
The surface area of the Ba ay is 1.95x 10? m?, with
a tidal prism of 9.4x10*m and a mean depth of
12.8 m (Environmental Study of Port Phillip Bay,
1973).
The hydrodynamics are characterised by (i) an
entrance region where fast ebb and flood jets (of
the order 3m.s !) dominate the circulation, (ii) a
flood-tidal delta, known as the Sands region,
where strong currents Occur in the major channels
and (iii) a large ‘inner’ region, where tidal flows
are weak (with an average of c.0.06 m.s) (Black
ct al., 1993). These circulation patterns are broad-
ly reflected by sandy bottoms in the faster flowing
regions and fine muds deposited in the centre of
the Bay. Sandy beds which predominate around
the margins reflect local wave activity.
Three commercially-dredged sites at Dromana,
Portarlington and St Leonards were selected for
the study (Fig. 1). Each site was located at a
similar depth (c.15m) in the ‘inner’ region, but
sites had different bed sediments and were ex-
posed to different current strengths and wave
attack.
At Dromana, bed sediments were dominantly
medium-fine sands with mean grain size (Table
1). A coarse fraction (0-1 phi; 0.5-1mm) was also
present (Fig.2). At Portarlington, bed sediments
were muddier (30.1% <63um, Table 1) but the
sediments also contained large numbers of shell
fragments and the overall mean grain size was
TABLE 1. Mean grain sizes, percentage mud and sand, and spring tidal currents at the field sites, The standard
deviations (SD) and the number of observations (N) are given.
SD (phi)
— Менә content
E | Domany | rz 0.33
0.44
Spring tidal
[к | стт
0.16
330
TABLE 2. Scope of field study and techniques applied
at Dromana (Drom), Portarlington (Port) and St
Leonards (SiL). The symbols show: *+* technique
applied; '-* not applied; 'P' planned.
Techni qué applied
Experimental дгедріп,
*
SEDCAM
Currents +
Seatevels — [||
+ Гр |
Sedimentanalyses | | — [| — |
-dedgepum — — — [| + | P | P |
[Comparison of dredges | - | - | P |
Drom | Port
de
+
+ |+
+ |+ [+
|
MEMOIRS OF THE QUEENSLAND MUSEUM
0.14mm. St Leonards sediments were
predominantly fine and very fine sand (Table 1).
The coarse fraction noted at Dromana and Portar-
lington (<Lphi; >0.5mm) was absent at St
Leonards (Fig.2).
Spring tidal currents are slower at Dromana and
Portarlington than at St Leonards (Table 1).
Dromana is the most exposed to wave attack and
Portarlington the most sheltered (Fig. 1).
FIELD MEASUREMENT TECHNIQUES
The investigations adopted a wide range of
techniques (Table 2) including some novel ap-
proaches.
DREDGING OF EXPERIMENTAL PLOTS
Experimental plots were established within
large areas (20-30km”, Fig.1) closed to all scal-
lop dredging during 1991, Supervised dredging
of these experimental plots by commercial scal-
lop fishermen was undertaken as part of a series
of controlled experiments designed to measure
the effect of scallop dredging on biological com-
munities (Currie & Parry, this memoir), bedform
topography, sedimentation rates and turbidity.
/ n» Ж j
| s
FIG.3. Jn situ sediment transport unit (SEDCAM). The underwater video camera and the sediment sensor
electronics are within the large housing. The smaller housing contains a battery power supply for the equipment
and underwater lights. An acoustic pinger (foreground) is included to assist retrieval, if the frame was
accidentally moved by a fishing boat,
SEDIMENT TRANSPORT AND DISTURBANCE PORT PHILLIP BAY 331
TABLE 3A. The location and recording times for current meters and tide gauges deployed in Port Phillip Bay,
1991. All times are Australian Eastern Standard Time (AEST). Water depths are not corrected for tidal level at
time of depth sounding. For recording intervals for S4 meters, SRB — special record blocks. For the measured
parameters, V—current velocity (i.e. speed and direction and/or E/W and N/S components); T-temperature;
C-conductivity; | I-vertical tilt of instrument; P=pressure (a coarse value only); D=fine resolution pressure. The
location of sites is shown in Fig. 1.
CURRENT METERS
| MHE
|o i cero E| |]
144° 44.883" E{ |
|o J 56410'E Dé eam] | |
ИШТҮҮ EE, iy ss
ir — |
| [14 —40910E | |
Experimental plots were 600x600m at
Dromana and Portarlington and 600mx750m at
St Leonards (Rosenberg et al.,1992). All plots
were dredged with the same intensity and dredg-
ing was continued until the entire plot had, on
average, been passed over twice by a scallop
dredge. The Portarlington plot was dredged with
this intensity on two occasions, three weeks apart.
To achieve the desired intensity of dredging, 5—7
commercial scallop boats worked each plot over
2—3 days. Dredging was always restricted to ap-
proximately 3 hours per day when tidal currents
were flowing strongly in the direction of down-
LAT. Е DEPLOYMENT | RECORDING TIME RECORDING
os LONG. | = ш TIME INTERVAL
z a E э © Mooring | Mooring i
2 = m a z deployed |recovered | record in | record in
= x IEE
= “3 EIZ
2 щ & < 158
2 zu —a ki ca
loses — seme
zii ur AGE Em
08. am 5 emm umm. nim
10. SMS -— prem. ийа aaa ane
À 3g? 2.80 KCN
18.8475 6/08/91 Peg
m aarti 2.70 | 12-09 m 10 uxo oe
EMIT E S 15/11/91 M ene өрө every 120 min. SRB
| aaa? 40.910°E every 120 min.
cor Ae ут! 38° 6. | 12:09 — 10 ЕЕ av. every 10
—— pre —425 96475 Е: 1/91 aue psum 1/91 E min.
(пре! AAND DT du — май. 15 min.
mar 15/11/91 ce нг изе
L| [|]
14: EA Font ah
14:00 1 sec. ау, for 1 min,
A EN. айас
every 45 min. SRB
every 90 min.
E EE sec. av. for 1 min.
every 15 min, SRB
every 15 min.
sms sec. av. for 1 min. |
every60 min. SRB
every 120 min.
—— De == sec. av. for ] min.
mx ae 30 min. SRB
every 120 min.
этип.
stream instrumentation. Relevant parameters
were measured by locating turbidity sensors, a
current meter and sediment traps downcurrent of
the experimental plots,
CURRENTS AND TIDES
Currents and wave orbital motions were
measured using an S4 electromagnetic current
meter deployed on a mooring (Table ЗА). These
vector-averaging meters are suitable for com-
bined wave and current environments. Sea levels
were normally taken from permanent stations
around the bay at Williamstown, Geelong and Pt
332
MEMOIRS OF THE QUEENSLAND MUSEUM
TABLE 3B. The location and recording times for video camera and sediment sensors deplayed on SEDCAM in
Port Phillip Bay, 1991. All times are AEST, Depths not corrected for tide at time of sounding. The location of
sites is shown in Fig. 1.
| ўпа За аа 1g
SEDIMENT SE} 5 ENSORS ___
WATER DEPLOYMENT TIME RECORDING TIME
SITE LAT/ RDING |
LONG. |DEPTH| Mooring Mooring. First record | Last record in INTERVAL
(m) deployed recovered in water water
StLA 38° 10.000'S 15:30 14:30 23:00 07:00 1 min, burst
144° 44.737'E 29/04/91 05/06/91 29/04/91 05/06/91 | every 8 hrs
ис 38° 010:528'5 14.5 13:30 15:10 16:00 08:00 1 min. burst
ME 44.483'E 15/07/91 17/08/91 15/07/91 17/08/91 every 8 hrs
RECORDING
INTERVAL
БУ | tnr ms
StLA | 38" Q0.000'S 0.11/ 15:30
ма? 44.737'Е 0.41 29/04/91
13:30
38? 10.5288
144° 44.883'E
15/07/91
PORT | 38°05.970's | 140 | O15) | 12:20
| 144° 40910E | __ 15/11/91
Lonsdale. An Aanderaa WLRS tide gauge was
deployed at Portarlington where tidal constants
and low frequency sea level measurements were
needed (Table 3A).
Tidal analyses, using the procedure of Foreman
(1977), Were applied to each current meter and
tide. gauge time series. The tidal components
could then be subtracted from the raw time serics,
leaving a residual current or sea level for separate
analysis.
In Situ SEDIMENT RECORDING INSTRUMENTATION
(SEDCAM)
Sediment transport rates under natural condi-
tions and during experimental dredging were
measured with SEDCAM (Fig.3), a free-standing
aluminium frame upon which were mounted à
Sony Video 8 camera, underwater lighting and 2
infra-red backscatter turbidity sensors (D&A In-
struments; Downing et al., 1981), All instruments
were mounted high enough to minimize any dis-
turbance to the bed. The camera was placed
c,60em above the bed. Filming was close enough
to the bed to view the onset of sediment entrain-
ment. Operating for | minute every 8 hours, up to
Mooring | Mooring
deployed | recovered
05/06/91 30/04/91 05/06/91 for 18 min. eve hous
г: 15:10 00:01 15:07
17/08/91 | 16/07/91 | 17/08/91
record
in water
record
in water
3min. av, of 30sec, scans
for 18 min. ever у юш,
18/1291 16/11/91 | 18/12/91 for 20 min, every hour
60 days of unattended operation was possible
with a 3-hour film.
In conjunction with the adjacent current meter,
SEDCAM enabled determination of the sediment
threshold in situ (with undisturbed sediments) by
inspection of the video film of the sea bed and
current meter measurements. SEDCAM also
records long-term variation in suspended sedi-
ment load, The equipment was originally
developed for studies of sediment movement in
wave/current environments in eastern Bass Strait
(Black ct al. in press).
The turbidity sensors were electronically con-
trolled and powered from within an underwater
housing so that no link to the surface was re-
quired. A bank of 6V rechargeable gel-cells
powers the system for up to 6 weeks. Turbidity
measurements were recorded at 30 or 60 sec
intervals and averaged every 3—4 minutes (Table
3B) using a 4-channel 128K Wesdata 692 data
logger.
The turbidity sensors were initially set 0.15—
0.45m above the bed, Divers checked the exact
elevations once the frame had settled into the
sediments (Table 3B). Calibration of the sensors
SEDIMENT TRANSPORT AND DISTURBANCE PORT PHILLIP BAY 333
Turbidity sensors were regularly cleaned by
divers to eliminate any marine fouling. However,
the large amount of drifting seaweed and the
activity of fish resulted in some interference with
the sensors, particularly at St Leonards.
Anomalous data were obvious from the excep-
tionally high readings and so these data could be
detected in the final calibrated time series.
SEDIMENT TRAPS
Sediment traps were used in addition to the
turbidity sensors to estimate time-integrated
sedimentation rates in natural conditions and
during dredging. Traps are particularly useful for
examining relative deposition rates; any altera-
tions to flow characteristics resulting from the
FIG.4. Bed sediment trap.
was essential (Appendix). SEDCAM and an ad-
jacent current meter were deployed 80 and 60m
downcurrent of the nearest boundary of the ex-
perimental plots at Portarlington and St Leonards
respectively. Equipment was deployed for 33
days at Portarlington and twice at St Leonards for
38 and 33 days respectively (Table 3B).
FIG.5. The sediment monitoring sled. Water pumps and turbidity sensors are placed on the upright at the front
of the sled. The controlling electronics are located in an underwater housing within the metal protective container
on the sled. Hoses terminate in metal honsings where water samples are collected in plastic bags.
334
FIG.6, ' Depth rings‘ being placed inthe bed to measure
depth of sediment disturbance by scallop dredges.
traps themselves should affect all traps similarly.
The traps record the total amount of sediment
deposited during a storm or during dredging.
Unlike the natural condition when sediment is
continuously entrained and deposited, sediment
entering a trap is not re-suspended. Accordingly,
the actual net deposition during a storm will be
less than that inferred from the trap results.
Each sediment trap consisted of twelve
transparent acrylic tubes (70mm diameter x
350mm height) standing vertically in a plastic
crate (Fig.4). The crate was normally placed on
the bed by a diver (hence sampling occurred at
0.35m). Additional traps, sampling at elevations
of 0.5, 1.0 and 2.0m, were placed on a supporting
metal frame at the Portarlington site to examine
vertical variation in sediment concentration and
grain size.
Sediment traps were placed 30, 60, 90, 200 and
400m downcurrent from the experimental plots,
as well as at two ‘control’ sites located up-cur-
rent. Traps were deployed just prior to the dredg-
ing, and removed às soon as possible after
dredging was complete. To investigate: the
relationship between natural deposition and
Water depth, bed sediment traps were also placed
along a transect perpendicular to the shoreline at
St Leonards in depths of 10-23m (Fig.1).
SEDIMENT MONITORING SLED
Sediment disturbance due to scullop dredging
was measured using a towed sled designed forthe
study (Fig. 5), Automated infra-red turbidity sen-
sors and clectric water pumps were attached at
0,25, 0.50, 1.14 and 2.00m above the bed and
towed successively at 5, 20 and 50m behind the
dredge. All instruments were electronically con-
trolled on the sled so that no electrical link to the
surface was required.
MEMOIRS OF THE QUEENSLAND MUSEUM
Turbidity sensors continuously-recorded fne
sediment concentrations during dredging opera-
tions, They provided a record of the sediment
concentrations as a function of elevation above
the bed and after different elapsed times, in ac-
cordance with the towing distance and boat
speed. Thus the measured concentration decay
rale (and the changing sediment grain charac-
teristics) enable determination of contours show-
ing sediment deposition behind the dredge.
Pumped samples of 2.5-3.51 of fluid were taken
adjacent to each of the turbidity sensors for
calibration. Pumps were controlled by a time-
delay magnetic switch, triggered at the surface
immediately before lowering the sled to the sea
bed. Pumps operated for 30sec after a delay of
4mins. Water sample bags, placed inside metal
housings on the sled, were replaced when the sled
was brought to the surface after each calibration
run.
Multiple boat Seen and depth to cable ratios
were tested while catch efficiency was also
monitored. The full range of calibration samples
(from each level, at each distance behind the
dredge, for each of the three measurement sites)
are being collected. We present the results from
trials at Dromana.
Devt of DISTURBANCE
Colour-coded ‘depth rings’ were used to deter-
mine the depth of bed sediment disturbed by
scallop dredges. Steel rings of 70mm diameter
were placed on the sea bed (excluding St
Leonards) and inserted 20, 40, 60 and 80mm
below the surface using a special tool (Fig.6).
33-38 of these sets of rings were placed at 34m
spacing diagonally across the dredge path before
cach experimental plot was dredged. The colour
and number code allowed observers on each ves-
sel to identify the rings caught by dredges.
POSITION FIXING AND WEATHER DATA
Instruments were deployed from the 2Um re-
search vessel ‘Sarda’ and located using a satellite
Global Positioning System (GPS). This was a
Furuno GP500 connected to a colour video plot-
ler. Hourly winds were taken from an exposed
anemometer on a low headland at Point Cock
(Fig.1). Barometric pressure and rainfail (3hr in-
tervals) were obtained from Laverton (Fig. 1).
NATURE OF THE BED
Bed sediment samples were collected by divers
using plastic corers, driven in by hand. In addi-
tion, replicate 0.1m? Smith-Melntyre grab
SEDIMENT TRANSPORT AND DISTURBANCE PORT PHILLIP BAY 335
-— =
Й DROMANA |
PORTARLINGTON
RE
B STLEONARDS
FIG.7. The sea bed at Dromana, Portarlington and St Leonards. The scale board is gridded at 2 cm intervals.
336
USD
S
o
MEMOIRS OF THE QUEENSLAND MUSEUM
VSD
FIG.8. Wave orbital motion time series at St Leonards (Deployment C). USD and VSD are respectively the
east/west and north/south components of the standard deviation of the wave orbital motion.
samples were taken at random within the ex-
perimental plots and 70ml subsamples taken for
sediment analysis.
Diver-operated underwater video and under-
water still (Fig.7) photography recorded ap-
pearance of the sites. The dredged and adjacent
sites, plus a control site, were filmed before and
$ 40.0
E 30.0
o
~ 20.0
a
m 10.0
© 0.0
20.0
15,0
& 0.0
n \
5.0
0.0
20.0
15.0
a
Ed 10.0
=
E Dredgin
У ging
E
ч
п.
л
15/7 17/7 18/7 19/7 20/7 21/7
5
E
N
œ
E
à
л
after dredging. (High turbidity prevented pre-
dredge filming at Portarlington.).
FALL VELOCITY AND EQUIVALENT GRAIN SIZE
Bed sediment, sediment trap and dredge plume
samples were analysed for fall velocities and
equivalent grain sizes. Percentage sands and
40.0
30.0
20,0
10,0
0.0
20.0 ; |
15.0
10.0 wav
5.0
0.0
SPEED (cm/'z)
uso
|
аа
00 г | | | | ]
15.0 a
a оа waves E
5,0 - E]
0.0
E E:
а o8 p Ї | ]
È otf
Ej L natural
xX 0.2 =
EN -
vi 50 |
25/7 26/7 27/7 2в/7 28/7 30/7
E
m
E
M
m
MX
=
Fu
л
FIG.9. A, Measured currents (SPEED), bed orbital motion (USD and VSD), and suspended load (SP1 апа SP2)
at St Leonards C. The SPEED is the total current speed. USD and VSD are respectively the east/west and
north/south components of the standard deviation of the wave orbital motion. SP1 and SP2 are the suspended
sediment loads at 0.15 and 0.30 m above the bed respectively
SEDIMENT TRANSPORT AND DISTURBANCE PORT PHILLIP BAY
STID (cms!
— Ы ou
е ©: a
coco
za
?
'
ES
(Т [ШИ
vsu
0.2
М » 7 | T aim +[
dredgings
9nu > natural natura!
oos E f
г | Ц |
1
15/12
SPA (leg m)
10/12 11/12. 19/12 13/12 14/12
1991
FIG. 9B. Measured currents (SPEED), bed orbital mo-
tion (USD and VSD), and suspended load (SP1) at
Portarlington (Deployment E). The SPEED is the
total current speed. USD and VSD are respectively
the east/west and north/south components of the
standard deviation of the wave orbital motion, SPI is
the suspended sediment loads at 0.15 above the bed.
muds were obtained by collecting the mud frac-
tion on a 63pm filter. Fall velocities and
equivalent grain sizes of the sand component
were measured in an automated settling tube con-
trolled by a Macintosh computer with software
from the University of Waikato (de Lange, pers.
comm.; Black & Rosenberg.1991),
Pipette analysis was used for the muds (Tuck-
er, 1988) and, to prevent destruction of the floc-
culated grains, all mud samples were kept and
analysed in sca water. Using split sediment
samples, fall velocities were much slower in fresh
than in sea water.
RESULTS
CURRENTS AND WAVES
Tidal currents, disru p ee by storms, dominated
the circulation at the 3 sites. However, the tidal
and wind-driven currents were usually insuffi-
cient to suspend sandy sediments at the sites on
their own. Additional wave orbital currents were
usually needed to initiate suspended sediment
transport. Peaks in the wave orbital currents
(Fig.$) occur every 7-10 days in synchrony with
the passage of high and low pressure systems at
these latitudes. The magnitude of the peaks near
the bed was determined by the water depth, wind
337
strength and wind direction. The latter deter-
mined the wind fetch and the resulting surface
wave height,
NATURAL CONCENTRATIONS
At St Leonards natural suspended sediment
concentrations during t the data collection period
were up to c.0. ikg. m^, although concentrations
of c.Ü.Ü2kg.ni? were more common (Fig.9A).
Similarly concentrations at Portarlington were
c.0.02kg.m" (Fig.9B).
The wind strengths during the measurement
periods were well above average (Fig.10), One
NNE wind exceeded 17m.s'! at Pt Cook which is
above the 98 percentile of all measurements made
over the period October 1987 to April 1989.
Although this wind has one of the longest fetches
in the bay for the St Leonards site, the measured
suspended suspended load at St Leonards dunng
this extreme event was <0. 1kg.m?. Similarly the
wind strengths at Portarlington during the meas-
urement period were commonly between 10-15
m.s! and blowing across some of the longest
fetches. Thus, the measured suspended loads arc
likely to be near the upper limit of the natural
levels.
CONCENTRATIONS DOWNCURRENT OF THE
EXPERIMENTAL DREDGING PLOTS
Concentrations recorded downcurrent of the
nearest boundary of [experimental dredging plots
were up to 0.2kg.m "at 60m from the St Leonards
plot and up to 0.07ке т at 80m from the Portar-
lington plot (Fig. 9A,B). Using the measured
velocities and assuming 60 and 80m excursions
of the plume, the dispersal times (between dredge
disturbance and measurement of the plume con-
centrations downstream) range from 6 to R
minutes at St Leonards and from 10 to 25 minutes
at Portarlington (Fig.9A,B). After this time, the
plume concentrations were about one order of
magnitude greater than the common natural
values of 0.02 Кеп.
Measurements directly behind the dredge are
available for Dagan only. There, concentra-
tions reached nearl y Sükg.m? 5m behind the
dredge; c.20 kg.m™ at 20m; and 12kg. m? at 50m
(Fig.11). For a boat speed of 3ms'!, the distances
represent elapsed times after disturbance by the
dredge of 1.7, 6.7 and 16.7secs.
COMPARISON OF DREDGE-RELATED AND NATURAL
SUSPENDED SEDIMENT LOADS
Sediment concentrations 17secs after distur-
bance are 2-3 orders of magnitude higher than the
338 MEMOIRS OF THE QUEENSLAND MUSEUM
20.0
Q
E 10.0
a
U оо ud Nfl муу, | |
c 360. — ! "
= m ENTE NL APTA Tr
5 180, | |
Ei |
а 0. [^^ wt - c n
18/7 23/7 30/7 5/8 13/8
1991
FIG.10. Wind speeds and directions measured at the Environment Protection Authority's Point Cook station
during the St Leonards deployment.
natural levels. The dredging-related concentra-
tions return to natural values recorded during
large storms after about 9 minutes. However, they
remain about an order of magnitude greater than
the more commonly recorded storm levels. The
elapsed time of 9 minutes is equivalent to 54m
from the dredge for prevailing current strengths
of 0,1m.s"!, Thus, elevated concentrations (equi-
valent to large storm events) and high sedimenta-
tion rates are restricted to within about 54m ofthe
dredge.
DEPOSITION RATES
Fall velocities (calculated assuming quartz
density in 20°C water) for the 30th, 50th and 70th
percentiles of the bed sediment grain size dis-
tribution for Dromana are 0.053, 0.039 and 0.027
m/s" respectively. The measurements indicate a
maximum plume elevation above the bed of
about 2m. A simple calculation serves to explain
the above results although more concise numeri-
cal modelling is being undertaken to treat the full
grain size distribution. In the absence of any
turbulence, the times for sediment to fall out of
suspension from 2m aboye the bed would be 37,
51 and 74secs for the 3 fractions noted above. The
turbulence behind a dredge would have the effect
of increasing these times, However, the calcula-
tion demonstrates that most sediment would fall
out of suspension within tens of metres behind the
dredge in currents of order 0.1m/s', even though
fine material may remain in suspension for much
longer times.
DEPTH oF DISTURBANCE
The depth of disturbance by dredges at the 3
field sites was indicated by the number of rings
captured from each depth within the sediment
(Table 4). While the overall capture rate was low,
highest numbers were collected from the surface
and the rings indicated that the maximum depth
of disturbance was 60mm at St Leonards and
40mm at Dromana and Portarlington. The results
suggest that the dredges dig further into the softer
sediments at St Leonards than in the coarser san-
dier sediments at Dromana. The relatively low
surface capture rates at Portarlington remain un-
explained,
The depth rings suggest that the ‘Peninsula’
dredge commonly-used in Port Phillip Bay typi-
"indivi 4. Number of depth rings recovered from each depth during experimental trials at each site.
ndividual rings from the same numbered set were collected in the same drag.
* The three rings collected on day 2 at Portarlington (X 4) were from the same numbered set, but the 2cm ring
was recovered by a different vessel from the other two.
Portarlington East X 2 | Portarlington East X 4
St. Leonards
d
E EISHHOHURUBHDRRRHORORE
Total FERE
вв о то
ECTLTUUEEZEJETEJETEIETETEIETETETETETETEIETETEYE!
SEDIMENT TRANSPORT AND DISTURBANCE PORT PHILLIP BAY
RADIENT:
INTERCEPT:
CORRELATION:
CONCENTRATION (kg.m-^3)
0.0 1.0 2.0 3.0
OBS-1 READING (Volts)
FIG.11. Sediment concentrations from water samples collected at 5, 20 and
50 m behind a Peninsula scallop dredge versus the turbidity sensor reading
at Dromana,
cally disturbs the top 10-20mm of sediment, but
that dredges sometimes disturb a layer up to
60mm thick. Predictions of sediment concentra-
tions based on these estimates of the depth of
sediment disturbance are similar to measured
concentrations. Measurements at Dromana indi-
cate that the plume extends about 1m above the
bed immediately behind the dredge. Sediment
concentration at the bed is c.1600 kg.m™, after
applying a pore volume correction factor of 0.6
to a density of 2650 kg.m^. Thus, if a layer of
sediment l.5cm thick is disturbed and
redistributed throughout a 1 m height, then it will
be ‘diluted’ 66 times, giving a sediment con-
centration of 24kg.m^, i.c. 1600/66. This meas-
urementis in accordance with that 5m behind the
dredge, „although measurements as high as
58kg.m? were observed (Fig.11).
DISCUSSION AND CONCLUSIONS
Measurements of the sediment concentration
hehind the dredge define the characteristics of the
plume and the depth of disturbance. This infor-
mation can be used to assess the magnitude and
з SLED AT Sm
* SLED AT 20m
* SLED AT 50m
339
spatial extent of sediment dis-
turbance by dredging and can
be generalised to other sea bed
types and grain size distribu-
tions. Thus, the measurements
can be used to assess the
potential environmental im-
pact of scallop dredging. Im-
pacts may be local physical
changes that directly impact
on biota (Currie & Parry, this
memoir) or far-field changes,
such as elevated turbidity that
may impact on seagrass orreef
communities.
Natural suspended sediment
concentrations during storms
were 2—3 orders of magnitude
smaller than the concentration
recorded immediately behind
a scallop dredge. The dredg-
ing- related concentrations
returned to natural storm
levels after about 9 minutes at
sites 60 and 80m downcurrent
of the nearest boundary of ex-
perimental dredging plots, al-
though the concentrations
were still nearly an order of
magnitude greater than those
occurring during the morc
common storm intensities. While a plume may be
visually observed behind a dredge for longer than
9 minutes, the plume at these times will consist
of fine sediments. Some of the finest sediments
may take a considerable time to settle; the timc
would depend on the prevailing weather and the
grain size. By disturbing the fine material, dredg-
ing may cause a significant redistribution of fine
sediments within the Bay. In addition, the dredg-
ing may break natural sediment bonds (cohesive-
ness and biological bonding), causing increased
likelihood of renewed suspension during natural
storms.
Any direct environmental impact of the plume
is likely to be small. However, the restricted
spatial extent of the bulk of the deposition will
result in localised high sedimentation. The meas-
urements provide a quantitative estimate of the
relative sediment concentrations during storms
and dredging. However, a number of com-
plexities in the natural system remain to be
teated. For example, wave orbital motion is a
strong function of water depth, and so more ener-
gy will be available at the bed for sediment
4.0 5.0
зап
entrainment around the margins of the bay than
in the deeper central regions. Indeed, quantities
caught by sediment traps in this program along a
transect with depths ranging from 10—23m were
about 50g in 10m depths compared with <10g in
water 15m or deeper, for the same time period.
We are now using the data to confirm wave
prediction theory and have established an annual
distribution of wave energies throughout the bay.
In conjunction with a numerical hydrodynamic
model of the tidal and wind-driven current speeds
(Black et 31,1993), a summary of the hydro-
dynamic energy available at the sea bed for sedi-
ment suspension at all sites in the bay is being
created. With the grain size data, this provides the
basis for an estimate of natural annually-averaged
suspended sediment loads for direct companson
with the dredging data.
ACKNOWLEDGEMENTS
Weacknowledge support from David Hatton in
data analysis. Thanks to Dave Byer (skipper),
Bob Metcalf and Tom Budd (engineers) and Matt
Hoskings and Mark Ferrier on the ‘R.V. Sarda’,
and to Tony Sheehan skipper of ‘R.V. Melita’,
plus all other crew members, for assistance with
field work. Thanks also to the many Marine
Science Laboratories staff members who
deployed and retrieved sediment traps, including
John Barry, Loren Brown, Anna Bury, Dave Cur-
rie, Rhonda Flint, David Forbes, Ross Haughton,
Matt Hoskins, and Geoff Nicholson. Mike For-
syth undertook analyses for sediment samples.
Andy Longmore collected water samples for ficld
calibration of the turbidity sensors. Lastly, grate-
ful acknowledgement to the skippers and crew of
the following commercial scallop boats involved
in experimental dredging trials: *A.B. Hunter',
‘A.B. Ventre’, ‘Conquest’, ‘Grace’, ‘Jennann’,
“Marie Lizette”, ‘Nephelle’, ‘Nimrod’, ‘Pegasus’,
‘Saint’, ‘Sandgroper’, 'Tingara’, and ‘Trinity’.
MEMOIRS OF THE QUEENSLAND MUSEUM
LITERATURE CITED
BLACK, K.P. & ROSENBERG, М.А, 1991.
Hydrodynamics and sediment dynamics in wave
driven environments. Volume 1, Field equipment
and data. Victonan Institute of Marine Sciences,
Technical Report 13: 1-229.
BLACK, K.P. & ROSENBERG, M.A, 1992. Natural
stability of beaches around a large bay. Journal of
Coastal Research 8(2): 385-397.
BLACK, K.P., HATTON, D.N., ROSENBERG, M.A.
1993, Dynamics of a large bay and ocean strait in
Southern Australia, Journal of Coastal Research
9(2): 509-538.
BLACK, K., ROSENBERG, M., SYMONDS, G,
SIMONS, R, & PATTIARATCHI, N.P. IN
PRESS, Measurements of wave, current and sea
level dynamics of an exposed coastal site. Interna-
tional Biennial Conference on Physics of Es-
tuaries and Coastal Seas.
CURRIE, D.R. & PARRY, G.D. THIS MEMOIR.
Preliminary analysis of the impact of scallop
dredging on a soft sediment community using
multivariate techniques.
DOWNING, J.P., STERNBERG, R.N. & LISTER,
C.R.B. 1981. New instrumentation for the inves-
tigation of sediment suspension processes in the
shallow marine environment. In Nittrover, C.A..
(ed.), ‘Sedimentary dynamics of continental
shelves’. Special Issue of Marine Geology 42:19—
34
ENVIRONMENTAL STUDY OF PORT PHILLIP
BAY , 1973. ‘Report on Phase Опе 1968-1971.
(Melbourne Metropolitan Board of Warks, and
Fisheries and Wildlife Department of Victoria;
Melbourne).
FOREMAN, M.G.G. 1977. Manual for tidal heights
analysis and prediction, Pacific Marine Science
Report 77—10. (Institute of Ocean Sciences,
Patricia Bay, Victoria, B.C.).
PARRY, G.D. & CURRIE, D.R. 1992. ‘Interim report
on the effects of scallop dredging on Port Phillip
Bay.’ Marine Science Laboratones Internal
Report 193.
ROSENBERG, M., BLACK, K, & PARRY, G, 1992,
Scallop dredging and sedimentation in Port Phillip
Bay. Volume 1: Sediment and hydrodynamic
measurements: Field data collection and analysis.
Victorian Institute of Marine Sciences Working
Paper 24: 1-78.
TUCKER, M. 1988, "Techniques in sedimentology.’
(Blackwells: Oxford), 394p.
SEDIMENT TRANSPORT AND DISTURBANCE PORT PHILLIP BAY
APPENDIX
CALIBRATION OF TURBIDITY SENSORS
Turbidity sensors were calibrated using the
techniques applied by Black & Rosenberg (1991)
using, for comparison, a bed sediment sample and
sediment captured in a trap elevated 0.35m above
the bed. Differences between the two calibrations
were related to differences between grain sizes in
the samples and so we adopted the calibration
using suspended sediments rather than the bed
sediments.
For validation, two sets of pumped suspended
sediment samples were obtained using a ' March‘
12V submersible electric pump attached to SED-
CAM by divers. These were located 60 m
downstream of experimentally-dredged plots.
The concentrations derived from pumped
samples and from sensors exhibit acceptable
agreement (Fig.12), particularly at Portarlington.
The larger deviation at St Leonards is probably a
sample handling effect, related to pre-drying of
samples, or a grain size effect. The grain size of
sediments captured in sediment traps during
storms used in the calibration will differ from that
observed downstream of the experimental dredg-
ing.
341
0.20 - —--
! ST LEONARDS
* OBS
— pump —|
CONCENTRATION (kg.m^3)
\
17th JULY 1991
|, PORTARLINGTON
0,04 —
CONCENTRATION (kg.m^3)
0.00
20/11 21/11
20th NOVEMBER 1991
FIG.12. Comparison of sediment concentrations taken
from pumped water samples and concentrations from
the OBS turbidity sensors at 35 cm above the bed. The
sites were downstream of experimental dredging
plots at St Leonards and Portarlington.
SCALLOP DREDGING; AN ENGINEERING APPROACH
P. COVER AND D. STERLING
Cover, P. & Sterling, D. 1994 08 10: Scallop dredging: an engineering approach, Memoirs
of the Queensland Museum 36(2): 343—349. Brisbane, ISSN 0079-8835.
An appraisal of dredges used in the southeast Australian scallop fishery was undertaken and
a comparison made with some scallop harvesting gear used elsewhere in the world.
Variations of the toothed mud dredge used in Australia were surveyed and described. Vertical
forces on the toothed mud dredge consist of downward directed hydrodynamic lift, weight,
and the upward component of the tow cable tension, These forces were analysed to show
how the resultant contact pressure changed with low speed. AMC flume tank and sea trial
measurements were used to produce a mathematical model for the horizontal forces, Turning
moments and dynamics during operation were analysed and modelled,
The toothed mud dredge was compared with the New Zealand dredge, Japanese Keta-amt,
and Scottish mini dredge for downward contact pressures and drag forces per meter of swept
width. The toothed mud dredge, keta-ami, and Scottish mini dredges exert high downward
contact pressures with point loadings, The toothed mud dredge had the highest drag while
the New Zealand dredge had the lowest drag especially at the lower tow speeds at which it
is normally used.
P.Cover and D. Sterling, Australian Maritime College, Р.О. Box 21 Beaconsfield. Tasmania
7250; 20 May, 1994.
Tasmanian, Victorian and Bass Strait scallop
grounds have seen an extensive period of
diminished returns and closures. The D'-
Entrecastreaux Channel was closed from 1970 to
1981 and again in 1986 (Perrin,1986), and the
Bass Strait Tasmanian zone was closed in 1987
(Zacharin, 1991). This scallop fishery is suffering
from low catch rates because of low stock levels
and poor гесгшітепі(р.Р.1., Tas. Sea Fisheries
data). The poor state of the fishery is partly at-
tributed to incfficient and destructive fishing
methods (McLoughlin et al, 1991).
Catching efficiency of the Australian scallop
'mud' dredge was found to be low: on average
only 11.6% (McLoughlin et al.,1991), and in-
cidental damage is high for the box type dredge.
High incidental damage may be detrimental to the
fishery’s long term viability (Zacharin,1988).
Scallop fishing gear used worldwide include
box type dredges, the ring mesh bag type dredge,
small multiple units and trawl gear. This gear has
evolved; each in its own part of the world to suit
arange of local conditions including scallop type,
bottom terrain, and local technology. There is
currently a drive to improve scallop harvesting
gear both in efficiency (catching and enginecr-
ing) and environmental impact.
To date there have been few studies of scallop
dredges from an engineering viewpoint. That re-
search includes work on: teeth and depressors
(Baird,1959), drag measurements (Hughes, 1973)
and the pressure drop behind a stalled foil (Vac-
caro & Blott, 1987). Baird (1959) found that teeth
improved catching efficiency and bottom contact
was improved by a depressor (or diving) plate.
Hughes (1973) measured typical bollard pulls
and warp cable tensions for box dredges in Port
Phillip Bay, Vaccaro & Blott (1987) suggested
that a simple flat depressor plate at 60—75 with a
gàp to chord length ration of 0.27 could improve
efficiency of scallop harvesting gear.
The Australian Maritime College (AMC) is
cooperating with CSIRO Division of Fisheries
and the Tasmanian Fisheries Department to re-
search better scallop harvesting gear; iis role ts to
investigate the engineering aspects of the gear.
The work conducted to date by the AMC had
its objectives to: 1. survey current box dredge
designs; 2, assess the engineering performance of
the box dredge; 3. compare the engineering
aspects of the box dredge design to designs used
elsewhere in the world,
METHODS
Box DREDGE ENGINEERING PERFORMANCE
A standard box dredge (Fig.l), as used in
Australia, was constructed and analysed in flume
tank and sea trials. Tests in the flume tank in-
volyed suspending the dredge in the flow by load
344
Skiss
Mar
y — I
^ \ р, =
P \—
Depressor plaw “Tew balle amaclunenz panis Front
Bykom єп WOL ur сийет bir hidden ander depressor plate
FIG.1. Schematic view of toothed mud dredge and
descriptive terms
cells. This gave a measure of the downward and
horizontal forces due to waterflow (hydro-
dynamic force). Sea trials allowed the total drag
(including ground effect) to be measured and
diver observations of the operational dynamics to
be made. The turning moments and dynamics in
operation were further analysed by mathematical
modelling data from the tests. Modelling was
based on equations of equilibrium applied to the
system of forces acting on the box dredge.
For equilibrium or no rotation: 1, the sum of the
moments about the teeth = 0 (moment arm dimen-
sions in Fig.2). 2, the sum of the vertical forces —
0. 3, the ground reaction forces at the Front,
Teeth, and Rear must always be greater than or
equal to zero.
Sum of the moments — (Tow force —
Hydrodynamic drag) x 0.285 — Weight x 0.413 —
Upward Component of Tow Cable Tension x
0.360 -- Hydrodynamic Downward Directed Lift
х 0.318 — Ground Reaction at the Front x 0.200 +
Ground Reaction at the Rear x 1.300
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n ow endo тешип.
“Row lorte
Hiyilrad ynanm stu.
hydro sain
down
Dredee ЕП
eic Hirn A Phe
(nud '
ieira Ст посі
I тасбит k
Lors un pe es an
hti
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FIG.2. Diagram of forces and lever arms acting on
toothed mud dredge.
MEMOIRS OF THE QUEENSLAND MUSEUM
FIG.3. The New Zealand dredge.
Sum of the Vertical Forces- Weight +
Hydrodynamic Downward Directed Lift — Up-
ward Component of Tow Cable Tension —
Ground Reaction at the Front - Ground Reaction
at the Teeth — Ground Reaction at the Rear.
Dynamic equilibrium may exist with the
ground reaction at front, teeth and rear varying
dynamically in response to the terrain, sea condi-
tions and other factors.
ENciNEERING CoMPARISON OF Box DREDGE WITH
DREDGES USED ELSEWHERE
Three dredges used elsewhere in the world,
obtained for comparison to the standard box
dredge, were: 1, the New Zealand dredge (Fig.3)
which comprises a flexible chain/ring mesh bag,
tickler chain and towing frame. In New Zealand
scallop fishing boats use a pair of ring hag
dredges upto 2.5m wide with heavy tickler chains
(Bull,1988); 2, the Japanese Keta-ami dredge
(Fig.4) contains a flexible ring mesh bag with
looped tickler chains mounted behind tines for
use on hard mixed and rocky ground. It is
designed to ride over rocky obstacles and not to
FIG.4. The Japanese Keta-ami dredge.
SCALLOP DREDGES, ENGINEERING APPROACHES
Wihysele d rowing beum for rone mter thee) pon
Tp of bay тг! T
"am
Fleve iT tag steel {лш к=
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BS
th | (|
ДА H |
1 Wh xS! ii
MU yt i
== à
FIG.5. The Scottish mini dredges.
pick up rocks (Zacharin, pers. com.); and 3, the
Scottish mini dredges (Fig.5) incorporate sprung
teeth for use on hard and rocky grounds and are
highly selective, not picking up rocks and other
debris (Franklin, Pickett & Connor,1980), The
Scottish mini dredges are normally towed in
gangs of three or more from a wheeled towing
beam,
The downward contact pressures applied by the
dredge components were calculated from
measured downward forces or weights of objects
divided by their sea bed contacting area.
Downward contact pressures have been com-
pared for the dredges analysed.
Total drag of each dredge was calculated from
warp tension measurements using a load cell
during sea trials. The catching width for each
dredge was direcily measured, Total drag for the
four dredge types were compared on the basis of
drag per meter of swept width.
RESULTS
SURVEY or Box DREDGE DESIGNS
In the SE Australian scallop fishery the local
fishers exclusively use a box type dredge in con-
junction with a dredge tipper. This is due to the
systems ease of operation and safe handling char-
acteristics. The current dredge known as the
toothed mud dredge (Gorman & Johnson, 1972;
Hughes,1972; Dix,1982) is a heavy (300+
150kg) steel structure composed of a steel mesh
box on skids with a bottom contacting tooth bar
or cutter bar and forward mounted depressor plate
which also serves as the attachment point for the
tow bridle.
The width of this dredge is about 3.3m but
varies from 2.2—4.6m to suit boat size, width of
the sorting tray and the vessel's towing capacity.
Fore to aft (length) dimensions vary only slight-
ly between dredges irrespective of width. Typi-
cally the measurement from the back of the box
to the tooth bar is about 1.2m. The box is 80—
100mm above the skids. The skid length is 1.5m
in atypical dredge, however forward extensions
as in the Peninsula dredge modification can add
up to 0.45m, while rear extensions of up to 0.3m
are often used. The forward extension of the skids
ts a modification designed to reduce the tendency
of the dredge to ride on its nose. The rearward
extensions should similarly reduce the tendency
for the dredge to ride on its rear.
A dredge height of 0.4m from bottom of skids
to top of the box is generally adopted. Short
stabilising fins are usually incorporated on each
side at the rear and add an additional 0.25m to
overall height,
The box type dredge generally referred to as the
toothed mud dredge is often used with a device
other than the tooth bar. In Port Phillip Bay it is
more usual to fit a cutter bar which does nol
protrude below the depth of the skids. Alterna-
tively a new device referred to as the ‘mouth
organ bar’ can be fitted. In Bass Strait or Tas-
manian waters the tooth bar normally has teeth
protruding 20-60mm below the skids. The teeth
are made from a hardened steel with the tips
trealed with hard face welding.
A variant of the box dredge known as the Bay
dredge, has a long history of use in Port Phillip
Bay. This dredge has the depressor plate set well
forward of the box and low to the ground, The
dredge ts normally towed at a 5-8 knots, In one
of the dredges observed, the cutter bar was angled
aft in manner which would not function at all in
digging up scallops from the sea bed, Intuitively
the Bay dredge relies on the hydrodynamic action
of the depressor to catch scallops.
On several dredges an old rubber tyre and
length of chain are towed from the top rear of the
box. This addition may hold the back of the
dredge in ground contact or serve some dynamic
purpose on rough or undulating terrain,
346
Лесі асе P7
FIG.6. Vertical forces acting on toothed mud dredge
modelled from lines of best fit of flume tank and sea
trial data.
Dredges which have seen extensive use show
high wear at the leading edges of the skids. In
most cases this wear zone is patched orreinforced
with hard facing weld. Dredges from Port Phillip
Bay which have been extensively used exhibit
thinning of the skids toward the rear. This may be
due to the dredges riding harder on the rear of the
skids when full or may be the result of using a
short tow cable or from repeated wear during
shooting away and haulback.
Box DREDGE ENGINEERING PERFORMANCE
Vertical forces. Vertical forces acting on an
operating dredge are: the weight, downward
directed hydrodynamic lift from the depressor
plate, and the upward component of the tow cable
tension.
Weight of the dredge is partially reduced by
buoyancy effects. The weight in water of the
standard dredge was measured by suspending it
in the flume tank by ‘load cell’ tension meters.
The hydrodynamic downward directed lift 15
the force exerted at right angles to the direction
of flow by the deflecting action of the depressor
plate. Hydrodynamic lift is generally proportion-
al to velocity squared and was measured over a
range of water speeds in the flume tank.
The upward component of the tow cable ten-
sion depends on the declination angle of the tow
cable and the total drag acting on the dredge. For
a completely ngorous treatment the weight and
hydrodynamic drag of the tow cable should also
be considered, Since the effect of cable weight
and drag are relatively small they have been
omitted for a more simplified view, The declina-
tion angle of the warp can be measured by an
MEMOIRS OF THE QUEENSLAND MUSEUM
inclinometer but for the shallow depths used in
the sea trials, straight line geometry can be as-
sumed. With this assumption the declination
angle can be derived from the cable length to
depth ratio used. The total drag was obtained
from sea trials by measuring the tow cable tension
over a range of tow speeds.
The net downward force is the sum of all the
vertical forces and must be greater than 0 for the
dredge to stay in bottom contact.
Mathematically these three components of the
vertical forces can be summed and analysed with
respect to speed as follows:
—The force due to weight (W) is constant.
—The hydrodynamic force can be expressed as:
1,= р А Суу? where v = velocity, A = area
of depressor, p= density of water, Сү = lift coef-
ficient of depressor.
-The upward component of the tow cable ten-
sion can be expressed as:
U = Total Dredge Drag x tan O Where O =
declination angle of the tow cable or alternatively
U =Total Dredge Drag x V cale length to depth ratio
-The net downward force (IN) is the arithmetic
sum of all the vertical forces. N=W+L-U
Fig.6 shows how the vertical forces on the
dredge change with speed.
Horizontal forces: The horizontal forces acting
on the toothed mud dredge are the tow force
(horizontal component of tow cable tension)
which 1s equal and opposite to the drag forces.
The total drag of the dredge is made up of:
hydrodynamic drag, friction and ploughing for-
ces.
The total drag was determined from sea trials
Mug Orua убт serae
Туспа А Mowing venena
Нун тта Fnrces (М)
Hubo eae
Speed tius]
FIG.7. Horizontal forces acting on toothed mud dredge
modelled from lines of best fit of flume tank and sea
trial data.
SCALLOP DREDGES, ENGINEERING APPROACHES
TABLE 1. Estimated downward contact pressures for scallop dredges and their components. For comparison
an 80kg man wearing size В shoes would exert a ground contact pressure of 26.1kPa.
in water) pressure
Toothed mud |310kg
(3050N)
(880N) Tickler chain
Chain and ring mesh bell
270kg Frame.
(2645N) Tines
Tickler chain
Scottish 510kg
(SO00N)
from measurements of tow cable tension over a
range of tow speeds. The hydrodynamic drag
component was measured in the flume tank by
suspending the dredge in the flow without bottom
contact. Friction and ploughing were calculated
as the difference between the total drag and the
hydrodynamic drag.
Hydrodynamic drag from the depressor plate
and from the rest of the dredge can be expressed
mathematically as:
D=1/2p A Cp ү?
velocity squared)
Friction is a mechanical force and can be con-
sidered to be independent of speed. The normal
expression for friction is:
F=pN p. = coefficient of friction:
(i.e. proportional to
nga
Tiedicl inslabulily wath
0 № стал sme v
Firmani Support Vae [N 1
Speed ims)
FIG.8. Ground support forces (Model 1) for toothed
mud dredge at a length to depth ratio of 8 on hard
ground..
kids (notin continuous contact) |extremely high | point loading; fore and aft rocking
Skids (in continuous contact) 12.7kPa
S
Toothed bar extremely high
1&3kPa
2.2-3.2kPa
3.9kPa
some bouncing
difuse loading
difuse loading
N = the normal force (at right angles to the
contacting surfaces).
In the case of an operational dredge the normal
force is equivalent to the net downward force.
Under the test conditions the net downward force
increases with speed (Fig.6), therefore we would
expect the friction force acting to also increase
with speed.
In the ploughing force from the teeth could be
a simple friction effect (i.e. independent of
speed). However since the rate of ground shear-
ing is determined by the speed of the dredge, this
could lead to the ploughing force being speed
sensitive.
From the trends observed in this sea trial and
flume tank data with respect to speed, the
horizontal force model (Fig.7) was developed.
Resultant forces and turning (rocking) moments:
Configuration of all the forces acting on the
toothed mud dredge (Fig.2) are such that a turning
moment may exist causing the dredge to rock
forward onto its nose orto rock back onto the rear
of the skids. Dredge wear patterns indicate that
this phenomenon commonly occurs,
The turning moments for two specific cases
have been derived from the experimental data and
the configuration of forces.
Modelling of all forces and resultant moments
obtained by resolving ground reaction forces on
skids and teeth show that where the cable length
to depth ratio is greater than 7 the resultant effect
suggests that the toothed mud dredge rocks for-
ward onto its nose (Fig.8). Where a short cable
length to depth ratio of 3 or less is used the dredge
348
would ride more heavily on the rear of the skids
(Fig,9).
Dynamic aspects: An observed feature of toothed
mud dredge performance on hard sand bottoms is
a pronounced pulsing in warp tension, This fea-
ture is affected by altering the warp length and
speed and is said to have some effect on catching
performance depending on the terrain. Sea tnal
warp tension measurements using a chart re-
corder have yielded pulse periods of the order of
two seconds with a variation in warp tension of
up to +30%.
Approaching this phenomenon from a theoreti-
cal point of view, we can consider the dredge as
having а moment of inertia (T) about a fixed point
(1.e. the tecth).
We can calculate I by:
1-Y mr where m — mass of component,
т = radius of gyration.
The forces operating are jn the form of disturb-
ing and restoring forces (or torques) (restoring
torque = т). The moment of inertia will be deter-
mined by the weight and shape of the dredge and
will be constant for a particular dredge, The res-
loring torque will vary with the angle of the
dredge (8) and therefore defining the restoring
torque is difficult.
A natural period must exist and will depend on
Ihe restoring lorque and the moment of inertia.
Т=2 (ry?
ENGINEERING COMPARISON OF Box WITH OTHER
Types or DREDGES
Downward vontact pressures: The downward
contact pressures for all four dredge types are
compared in Table 1. The teeth of the toothed
mud dredge and the skids 3f not in continuous
contact will exert extremely high bottom contact-
ing pressures. The New Zealand (Fig.3) dredge
exhibits low contact pressures for all its bottom
contacting elements, The Keta-ami (Fig.4) exerts
extremely high bottom contact pressures at the
tynes but low elsewhere. The Scottish mini
dredges (Fig.5) also exert extremely high bottom
contact pressures at the teeth but low elsewhere.
Comparison of drag forces per m of swept width:
The toothed mud dredge has the highest drag per
meter of swept width and the New Zealand
dredge has the lowest drag (Fig.10).
The toothed mud dredge has the highest level
of ground shear and friction type drag as well as
the highest hydrodynamic drag. The ground shear
and friction can be estimated by the y intercept
MEMOIRS OF THE QUEENSLAND MUSEUM
m qm
—— den
To kw
Treated rug pert Fine UND
jreaEEEHEEBRHNEHDECOUEDLZUEEGEHEHEEEGCEIHEBSSHHSHBE
1100:
LU | 1
Бк! (ти!
FIG.9. Ground support forces (Model 2) for toothed
mud dredge at a length to depth ratio of 3 on hard
ground.
on the drag curve (Fig.10). The hydrodynamic
component is evident in the amount of increase
in drag over the specd range.
The New Zealand dredge has significantly
lower ground shear and friction than any of the
other dredges. Its hydrodynamic drag component
is however almost as high as that of the toothed
mud dredge.
The low hydrodynamic drag of the Ket-ami and
Scottish dredges reflect their low frontal area and
absence of depressor plate devices,
DISCUSSION
The Bay dredge depressor plate approximates
the criteria cited by Vaccaro & Blott (1987) for
optimising the pressure drop behind a stalled
horizontal wing in proximity with the ground. It
is possible that this dredge is a hydrodynamic
scallop catching device that has evolved over a
period of time by trial and error.
The net downward force on the standard dredge
is fairly high (greater than 2000N or 204kg)
which should be more than adequate to maintain
good bottom contact. Under normal operating
conditions the net downward force will actually
increase with speed and this should ensure better
than necessary bottom contact.
At low speeds (up to 3 knots) the greatest source
of drag on the toothed mud dredge is from friction
and ploughing. At higher speeds (5-8 knots) the
hydrodynamic component becomes dominant
and is the component which will limit the towing
speed, For the dredges using a cutter bar or
mouthorgan bar (which does not protrude below
SCALLOP DREDGES, ENGINEERING APPROACHES
(inthe Mad Ord ee
Seul liy pede
нир quem efe ww lille 0%
Ie eain olivate
Bye ws)
FIG.10, Comparison of dredge types with repect to
Weir drag per m of swept width.
the skids) the friction and ploughing component
of drag will be much reduced.
The box type dredges have a definite tendency
to ride on the front or rear of the skids. This can
be controlled by the warp length to depth and tow
specd, or alleviated by the design modifications
of skid extensions fore and aft. The tendency of
the dredge to rock fore and aft may contribute
advantageously or adversely to catching perfor-
mance. It could be controlled to some extent by
changes in warp length to depth, tow speed or the
friction and ploughing forces (by altering tooth
penetration). The addition of a rubber tyre and
length of chain should help to reduce the tenden-
cy of the dredge to ride on its nose and could help
to damp out fore and aft rocking.
Although the average downward contact pres-
sure exerted by the toothed mud dredge is
reasonably low, the point loading and dynamic
action might mean that very high intermittent
contact pressures will occur, The averagc
downward contact pressure of the ring mesh bag
and the other dredge types is very low and not
likely to vary to any large degrec.
The teeth and cutter bars of the toothed mud
dredge, the tines of thc Keta-ami and the sprung
teeth of the Scottish dredge will exert a high point
loading. Very high contact pressure is likely to
contribute to damage of the catch and damage to
the environment.
In terms of drag, the best performer was the
New Zealand dredge. It had the lowest cost in
terms of total drag at its operational speed of 3
knots. The toothed mud dredge performed poorly
due to a much higher drag especially at 5-6 knots,
LITERATURE CITED
BAIRD, R.H. 1959, Factors affecting the efficiency of
dredge. In Kristjonsson, Н. (ed.), 'Modem fishing
gear of the world 1'. (Fishing News Books; Lon-
don).
BULL, M.F. 1988. The New Zealand scallop fishery: a
brief review of the fishery and its management.
Pp.42-50. In Dredge, M.C.L., Zacharin, W.F, &
Joll, L.M. (eds), *Proceedings of the Australasian
Scallop Workshop, Hobart’, (Tasmanian Govern-
ment Printer: Hobart).
DIX, T.G. 1982. ‘Fishery situation report 8. Scallops”.
(South Eastern Fisheries Committee, C,S.T.R.O.
Marine Laboratories: Cronulla),
FRANKLIN, A., PICKETT, G.D. & CONNOR, Р.М,
1980, The scallop and its fishery in England and
Wales. Ministry of Agriculture Fishing and Food,
Laboratory Leaflet 51.
GORMAN, Т.В. & JOHNSON, H.T. 1972. * E.R.V.
Kapala cruise report no 5'. (Chief Secretanes
Department, N.S.W, State Fisheries: Sydney).
HUGHES, W.D. 1972. Scallop dredging gear and
methods. Australian Fisheries July: 12-15.
HUGHES, W.D. 1973. Operational tests on Victorian
scallop boats. Australian Fisheries May: 14-16.
MCLOUGHLIN, RJ, YOUNG, P.C, MARTIN, R.B.
& PARSLOW, J, 1991, The Australian scallop
dredge; estimates of catching efficiency and as-
sociated indirect fishing mortality, Journal of Fish
Research 11: 1-24,
PERRIN, R.A, 1986, The D'Entrecastreaux Channel
scallop fishery: its past present and future. Master
of Environmental Studies Thesis, University of
Tasmania. (Unpubl.).
VACCARO, MJ. & BLOTT, A.J, 1987, Scallop gear
selectivity studies: hydrodynamic modifications.
Narragansett Laboratory Reference Document
87-27, (National Marine Fisheries Service,
Northeast Fisheries Centre, Fisheries Engineering
Group, Narragansett, Rhode Island).
ZACHARIN, W.F, 1988. Alternative dredge designs
and their efficiency, In Dredge, M.C.L., Zacharin,
W.F. & Joll, L.M. (eds), ‘Proceedings of the
Australasian Scallop Workshop, Hobart’. (Tas-
manian Government Printer: Hobart).
ZACHARIN, W.F. 1991, Slow recovery for Bass Strait
scallops. Australian Fisheries, January 1991.
IMPROVED HATCHERY AND NURSERY REARING TECHNIQUES FOR PECTEN
FUMATUS REEVE
М.Р. HEASMAN, W.A, O'CONNOR & A.W, FRAZER
Heasman, M.P., O'Connor, W.A. & Frazer, W.A, 1994 08 10: Improved hatchery and
nursery rearing techniques for Pecren fumatus Reeve, Memoirs of the Queensland Museum
36(2): 351-356. Brisbane. ISSN 0079-8835.
Fortnightly sampling of a population of the hermaphroditic scallop Pecten furatus in Jervis
Bay was initiated in July 1991. Results over the first 18 months showed that wild stocks
constitute a poor and unpredictable source of ripe ready-to-spawn broodstock for hatchery
use. This prompted development of hatchery conditioning protocols. The most rapid develop-
ment of ponads occurred when broodstock were held at 15°C and fed to satiation (6x10? cells
scallop"! day”! ona diet of approximately equal amounts of at least 3 of 4 microalgae, namely:
Chaetoceros calcitrans, Pavlova lutheri, Tahitian Isochrysis and Chroomonas salina),
Intragonadal injection of serotonin at 0.05m] of a 0,5x (F^N solution per scallop reliably
induced sperm release within 5-25min over a broad (12-24°С) temperature range. Survival
from fertilisation to D-veliger stage was substantially improved by incubating eggs in
suspension at up to 100 mI"! in aerated cylindro-conical vessels. Survival to metamorphosis
on Day 16 ranged from 5-20%, Rates up to 7096 were achieved with experimental scale
cylindro-conical rearers when scawater was prefiltered to 1m or when antibiotics were used,
Post-settlement retention rates of 10-50% were achieved by transferring pediveligers onto
cylindrical downweller screens fitted with [60j.m polyester mesh. Growth of 5—10mm
juvenile scallops maintained in an upweller nursery unit located at a site at the entrance to
Port Stephens was found to increase with increasing scawater flow rates up to 40ml рг!
biomass min! and to be suppressed when the surface area of scallops approached 100% that
of the screens on which they were stocked, Mean growth rates of 2.8 mm week! were
exhibited over the size range 5-25mm when maintained at low density in screens or lantern
cages suspended from a long line at 20-24°С, Small spat in the hatchery grew faster with
increasing temperature in the range 12-27°С but ceased growing at 1.5-3пут,
M. P, Heasman, W. A. O'Connor, and A. W. Frazer, NSW Fisheries, Brackish Water Fish
Culture Research Station, Salamander Bay, New South Wales 2301; 9 March 1994.
The NSW scallop fishery 1s spasmodic and
confined to Jervis and Twofold Bays (Fig.l).
Peak annual catches of 1000-3000 tonnes live
weight occur only once in 10 years with insig-
nificant catches in intervening years (Fuentes et
al., 1992). If higher and more consistent scallop
yields are to be achieved in NSW, the central
problem of low and variable annual recruitment
of juveniles, must be addressed. Wild caught
Pecten fumatus spat in Jervis Bay (and probably
elsewhere in southern NSW) is likely to be low
and unreliable in most years (Fuentes et al., 1992),
The importance of reliable, low-cost hatchery
and nursery rearing techniques for P, fumatus and
a successful pilot hatchery trial in May 1989
prampted à 3 vear, Fishing. Industry Research
and Development Corporation (FRDC) funded,
research project at the Brackish Water Fish Cul-
ture Research Station (BWFCRS) from July
1991.
The 1989 pilot study gave encouraging results
using wild scallops from Jervis Bay as spawning
stock and conventional hatchery rearing techni-
ques and equipment (Frankish et al., 1991). Ap-
proximately 6 million settled spat were being
produced at estimated survival rates from spawn-
ing to D-veliger (Ist feeding) stage of c.60% and
from D-veliger to post-settlement, of c.7096.
Several hundred thousand settled spat were
retained and onreared to H0—20mm shell height at
a similar rate of survival (Frankish et al,,1990),
These results contrasted with those previously
attained by Tasmanian oyster hatcherics using
comparable techniques and equipment, In at-
tempting to meet Tasmanian govemment con-
tracts for the supply of 4.2 million P. fumatus
juveniles in the range 10-20mm, the hatcheries
were only able to supply 100000 and 280000 in
1987 and 1988 respectively, Up to this time, the
largest spawning of P. fumatus had produced 125
million eggs but no hatchery had produced more
than 500000 settled spat from one batch of larvae
(Cropp & Frankish, 1989),
From the outset of this project, in 1991, it was
considered that previous high variability in
hatchery success with P. fumatus could have
352
Ice
' N
ass-
NSW Port Slephens
Sydney
Jervis Bay
mws-
4 Twolold Bay
VIC 2 Y
FIG, i, Central and southern New South Wales-
arisen through one or a combination of several
factors. These included: variability in the quality
of eggs sourced from wild spawners; subtle, but
critical differences in equipment and techniques
employed, especially in relation to settlement and
metamorphosis of pediveligers; disease(s) and
larval nutrition factors.
MATERIALS AND METHODS
EVALUATION OF WILD STOCKS OF P. FUMATUS AS А
Source or READY TO SPAWN Вроор=тоск
Fortnightly sampling of a Jervis Bay population
of P. fumatus was initiated in July 1991. On each
occasion 120-150 scallops in the range 55-90
mm shell length were collected by a professional
diver. All collections made between 7-8am were
road freighted (insulated from an underlying
layer of ice which maintained them at 10-15°С)
to BWFCRS within 8—-LOhes, They were immedi-
ately stocked into a lantern cage suspended in à
1000] holding tank at ambient temperature (16—
22°C). The following morning 40 randomly
selected scallops was measured, subjected to a
macro-visual staging of gonad condition (Fuentes
et 31,1992) and then dissected to determine
gonad somatic index (051),
Weight of gonad
= Total shell free drained vega ee
Of the remaining 80-110 scallops, the 10 in-
dividuals exhibiting highest apparent gonad con-
dition (degree of ripeness) were subjected to
induction of spawning stimuli within 72h of cap-
ture. Induction of spawning stimuli comprised the
exposure of scallops to 3 thermal cycles in which
MEMOIRS OF THE QUEENSLAND MUSEUM
temperature was raised 3-8°C above ambient
over periods of 45—60 minutes.
Release of sperm and eggs was recorded for this
hermaphroditic species with fecundity being
determined in the case of egg releases.
GoONAD CONDITIONING PROTOCOLS FOR CAPTIVE
Broopstock
Attempts to condition broodstock in the
hatchery were conducted between July and Sep-
tember, 1991, Wild scallops with medium to high
gonadal development attained a ripe, ‘ready to
spawn’, condition in 4—6 weeks during July and
August at 16-19°C and fed to satiation. Gonad
condition regressed as temperatures rose above
20°C in September and October.
These results prompted construction of a con-
trolled temperature broodstock conditioning
facility at the BWFCRS to develop techniques
that would enable controlled ripening, stockpil-
ing and induced spawning of captive broodstock
throughout the year, This facility, commissioned
in April 1992, comprised 4 water baths held at
12.0+0.5; 15.0+0.5; 18.0+0.5 and 21.0+0.5°С
respectively, each accommodating 36 x10 1 plas-
tic aerated aquaria to accommodate individual
scallops. Experiments to identify appropriate
microalgal diets and satiation feeding levels were
initiated in June 1992. Subsequent trials to idën-
tify optimum combinations of holding tempera-
ture at 100, 50, 25 and 12% of satiation feeding
levels were conducted in July 1992.
SPAWNING INDUCTION AND INCUBATION PROTOCOLS
A series of trials was conducted to determine
whether intergonadal injection of the neuro-
transmitter serotonin is more effective than
temperature shocks in triggering spawning of ripe
P. fumatus, To establish optimum dosage rates,
0.05ml serotonin was injected at concentrations
of 10 to 107 №. Time to spawning of sperm and
eggs was recorded as was fecundity in the case of
released eggs. Fertilised eggs were stocked into
П eylindroconical vessels and incubated at den-
sities of 1-100 eggs ml? to gauge the effect of
stocking density on survival to the 'D' veliger
stage 48h after fertilisation,
LARVAL REARING TECHNIQUES
Standard hatchery techniques and equipment
for Sydney rock oysters (Saccostrea commer-
cialis) larvae (Frankish et al.,1991) were used
over the first 12 months. Consistently poor
hatchery survival achieved by these means and
lack of suitable facilities with which to conduct
HATCHERY AND NLIRSERY TECHNIQUES FOR PECTEN FUMATUS
replicated trials to evaluate altermative rearing
techniques, prompted construction of a small
scale bivalve larvae rearing system in August
1992. Ten standard 10001 flat-bottomed cylindri-
са] oyster larval rearing vessels were utilised as
controlled temperature baths, each accommodat-
ing 4x801 cylindroconical rearers, As with the
10001 vessels, the smaller units were made of
rotationally moulded polyethylene.
Experimental treatments compnsed four Lypes
of seawater preparation te. filtration to 0.2m
absolute; filtration to 1.Ошт nominal; filtration to
1.Орт nominal plus 10mg Г! chloramphenicol
and an unfiltered scawater control. These were
combined factorially with two alternative diets,
namely, a standard blend of 3 microalgal species
(C. calcitrans, Tahitian /'sochrysis, and P. luther?)
and the same blend of microalgae concentrated
into a slurry by centrifugation and stored for 1-6
days prior to feeding. The chloramphenicol treat-
ment was to evaluate the claim that a closely
related European scallop P. maximus, cannot be
hatchery reared with consistent success without
use of such powerful broad spectrum an-
timicrobials (Samain et aL,1992) and hence
whether microbial pathogens posed a significant
constraint to hatchery success with this species.
Inclusion of chloramphenicol as an experimen-
tal treatment should not be construed as an endor-
sement of 115 use. To the contrary, identification
of alternatives to use of broad spectrum anti-
biotics that will ensure consistently high survival
of hatchery reared P. fumarus has become the
most important single objective of this project.
LARVAL SETTLEMENT AND EARLY NURSERY
REARING PROTOCOLS
Post settlement recovery of P. fumatics
pediveligers using conventional plastic mesh
catch (culch) materials and by transferring
pediveligers directly onto downweller screcns
fitted with 160j.m polyester mesh were com-
pared. Subsequent growth and survival was
monitored for post-larvae retained on down-
weller screens in the hatchery and for those trans-
ferred to upwellers screens and lantern cages al
Tomaree Head, adjacent to the mouth of Pon
Stephens (32° 44'5; 152° 11'E; Fig. 1).
A trial to determine optimum stocking and
water flow rates for small juvenile Р. fumatus
reared on upweller screens at Tomaree Head was
initiated on Christmas Eve 1992. A total of about
20000 spat averaging 5.6mm and 30mg were
stocked at 8 different densities (5 replicates per
density) using 40 miniaturised upweller umts
353
each consisting of vertically nested stacks of 8
interlocking screens.
The salinity tolerance of 1-2mm juvenile scal-
lops was investigated in the laboratory as were
interactive effects of salinity and temperature on
growth and survival.
SPECIALIST HANDLING TECHNIQUES FOR EARLY
JUVENILE AND ADULT SCALLOPS
Mechanical methods such as seawater jets and
scrapers used to dislodge small (10mm) bysally
attached Р, fumarus from culch materials and
nursery scrcens were found to cause injury and
subsequent high mortality. To address this prob-
lem, the effectiveness of a number of irritant
chemicals and physiological stress factors to in-
duce detachment of 2-4mm juveniles was
evaluated in fully replicated trials,
A comparable senes of trials was also con-
ducted to test the anaesthetic properties of a range
of chemical compounds on mature P, furatus of
55—75mm shell height. The aim was to identify
quick and simple techniques of reducing stress
and subsequent inadvertent spawning caused by
routine handling and assessment of gonad status
of hatchery conditioned scallops.
RESULTS AND DISCUSSION
Упр Stocks or Р, FUMATUSIN NSW as à SOURCE
oF Ripe BROOD STOCK
Fortnightly sampling of a Jervis Bay population
of P. fumatus (July 1991-December 1992)
revealed that this potential source of ripe ‘ready
to spawn’ broodstock is nearly always unproduc-
tive and unpredictable. Even when breeding con-
dition was highest, as indicated by peak in mean
gonad-somatic index values of 18-21%, very few
individual scallops, including those in à ripe con-
dition (large, turgid, glossy and richly coloured
gonads) responded positively and predicably to
conventional spawning induction stimuli, Of 300
(1O/collection) apparently ripe scallops subjected
to spawning induction stimuli over the first 18
months of the project, less than 4% were success-
fully induced to spawn eggs. Moreover. seasonal
peaks in breeding condition, were not consistent
from year to year, For example, recorded mean
GSI values Were highest between December 1991
and March 1992 (Summer) but were continuous-
ly low during May-September 1992 (late
Autumn-mid Spring). This pattern varied from
that of chronically low mean GSI values recorded
by Fuentes et al, (1992) through Summer and
early Autumn of the two previous years and coin-
^
cided with unseasonally high winter sea tempera-
tures and unseasonally low summer sea tempera-
tures.
HATCHERY CONDITIONING PROTOCOLS
Experiments to jdentify appropriate microalgal
diets, feeding rates and temperatures for gonadal
growth were undertaken in June 1992. Results of
microalgal clearance rate experiments indicated
that Pavlova lutheri, Tahitian Isochrysis aff, gal-
bana, Chroomonas salina and Chaetoceros
gracilis are ingested by adult scallops at similar
rates at 14, 18 and 21°С but more slowly at 11°С.
Satiation feeding rate using diets containing ap-
proximately equal numbers of cells of these 4
species, was estimated as about 6x10? cells per
day for broodstock of 55—75mm shell height. Cell
densities of Tetraselmis suecica, another microal-
gacommonly used to feed bivalves, declined over
the first 8h of the experiment but then fluctuated,
indicating resuspension of undigested cells from
the faeces,
In a subsequent 6 week conditioning experi-
ment, egg production rate associated with inad-
vertent spawning, along with gonad size and
condition factors, were found to be highest when
broodstock were held at 15°C, lowest for scallops
maintained at 21°C and of intermediate values at
12°C and 18°C. Across all these temperatures,
feeding of individual scallops with twice daily
algal rations of 3x10? cells (100% satiation) and
1.5x10? cells (50% satiation) produced higher
gonad ratings and egg production rates than ra-
tions of 0.75x10" cells (25% satiation) or
0.375x 10? cells (12.546 satiation),
Frequent inadvertent spawnings triggered by
handling emphasised the need for conditioning
equipment and protocols that minimise handling
and otber disturbance factors. No inadvertent
spawning however, occurred at feeding rates of
25 or 12.5% of satiation at 12°C, Use of low
temperatures combined with reduced feeding
rates might therefore enable stockpiling of
broodstock at prime reproductive condition.
Opportunistic usc was made of near ideal water
temperatures (14—16°C) in August and early Sep-
tember 1992 to candition 100 broodstock. Scal-
lops were held in lantern cages suspended in
20,0001 tanks in the bivalve hatchery at BWFCRS
and fed to satiation on a diet comprising equal
amounts of the previously cited 4 microalgal
species. To reduce the incidence of inadvertent
spawning, rations of microalgae were drip fed
into fhe tanks. The impact of water changes was
kept to з minimum and handling of stock was
MEMOIRS OF THE QUEENSLAND MUSEUM
totally avoided. No inadvertent spawnings were
recorded over this period.
After 4 weeks of conditioning, 30 scallops were
randomly selected and subjected to attempted
thermal induction of spawning. Of these, 10
spawned as males and 12 as females, the latter
yielding 25 million eggs and thence 13 million
D-veliger larvae,
SPAWNING INDUCTION, FERTILIZATION AND
INCUBATION PROTOCOLS
Attempts to induce spawnings im ripe P.
Jumarus sampled fortnightly from Jervis Bay,
were almost always unsuccessful, This result was
originally ascribed, at least in part, to inadequacy
of a standard thermal spawning induction
stimulus (exposure of scallops to successive
temperature nse cycles of 3-8"C above ambient)
to trigger spawning. This misconception was cor-
tected when development of effective gonad con-
ditioning protocols yielded broodstock that
spawned viable eggs rapidly in response to the
same thermal induction techniques.
A dose of 0,05m] intragonadal injection of a
0.5x109N serotonin solution induced sperm
release Within 5-25 minutes over a broad range of
temperature (14-22°C) in scallops with moderate
to high gonad development. As with thermal
shock technique however, induced spawning of
eggs using serotonin was found to be effective
only in suitably conditioned broodstock.
Injection of serotonin was nevertheless found
to have a distinct advantage over the thermal
induction of spawning. In being able to induce
spawning of individual scallops held in isolation
from one another. This in turn enables better
control over the timing and extent (sperm to egg
ratios) of fertilisation and the reduction of self
fertilisation in hermaphoditic species such as P.
fumatus, This attribute may prove particularly
useful if applied to induced triploidv programs in
the future. Use of serotonin induction of spawn-
ing has heen extended to Sydncy rock oysters at
the BWFCRS in anticipation of Ihis application,
Results of preliminary incubation trials con-
ducted in November 1992 indicated that survival
from fertilisatiun to D-veliger can be substantial-
ly improved by use of aerated cylindroconical
vessels in Which eggs are kept in suspension
rather than allowing them to settle in a monolayer
on the floor of conventional flat bottom oyster
larvae rearing vessels, Suspended incubation also
enables eggs 10 be stocked at high densities (up
to 100 mI") without apparent impairment to sur-
vival.
HATCHERY AND NURSERY TECHNIQUES FOR PECTEN FUMATUS
| i € —
SD — —
toin all
1 E 5 E
tet
R
беду (Ж).
тне АМА Lum].
6
n
ro oR о Gd H d» т oW 15
туу Imm артат
FIG.2, Summary of the growth and survival of
hatchery reared scallops (Pecren fumatus) at
BWFCRS, Salamander Bay. Dotted lines represent
the results of the May 89 larval batch. Shaded areas
indicate the range of results obtained from 6 larval
batches conducted since July 91.
LARVAL REARING TECHNIQUES
Larval rearing trials conducted over the first
year of this project (Fig. 2) used standard 10001
flat-bottomed cylindrical tanks, used previously
at the BWFCRS for the hatchery production of
Sydney rock oysters (Frankish etal.,1991) and for
the pilot hatchery production of P. fumatus in
May 1989 (Frankish et al.,1990).
Survival to metamorphosis (Day 16-18) was
520% compared very poorly with the May '89
result but was in keeping with earlier results
achieved by commercial hatcheries in Tasmania
in 1987-1988 (Cropp & Frankish,1989). Larval
growth rate varied and was not correlated with
survival. Metamorphosis was attained at shell
height of 225-240j.m 14—20 days after spawn-
ing.
The first trial to systematically address the
problem of low hatchery survival was undertaken
1n October 1992. Survival rates from D-veliger
stage to the onset of metamorphosis (Fig. 3)
varied with method of seawater preparation,
being highest (70-80%) with chloramphenicol
treated seawater and lowest (less than 10%) for
0.2,.m filtered and unfiltered seawater. Larval
survival rates were reduced but growth rates en-
hanced by algal concentrate diets, Subsequent
patterns of survival through metamorphosis were
however different with highest retention rates of
30-40% with lpm filtered seawater and lowest
rales of less than 1% with both unfiltered and
0.2m absolute filtered seawater treatments.
These results highlighted the importance of
continued research to identify seawater prepara-
tian, management protocals and feeding/stocking
regimes that will enable consistent attainment of
commercially acceptable hatchery survival rates.
These are generally considered as nett yields of
0.2-1 of settled spat ті! of rearer volume (L.
Goard, pers. comm.). The major challenge faced
by continuing research is achievement of satis-
factory vields of settled spat without the use of
antibiotics.
LARVAL SETTLEMENT AND EARLY
Nursery REARING PROTOCOLS
Of 7 larval rearing cycles completed during the
first half of this project, 3 yielded significant
numbers of spat. Approximately 15,000 spat
were produced in November 1991; 30,000 in
March 1992 and 200,000 in October/November
1992, Survival rates through settlement varied
over the broad range 5-5096. Lowest survival was
associated with the use of traditional plastic mesh
culch materials deployed directly into larval rear-
ing vessels, Much higher (10-50%) survival rates
have however been consistently achieved by
transferring pediveligers into downweller screens
fitted with 160pm polyester mesh just prior to
settlement.
Once settled onto downweller screens, sub-
sequent mortality of spat was negligible. Growth
of spat maintained in the hatchery is highly
temperature dependent, increasing from c.15-
150j.m.day with rising temperature over the
range 12—27°С. Growth abruptly stalled as
hatchery held spat attained a size of 1.5-3mm.
P. fumatus spat transferred to a longline unit at
= A
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mm d 7 Chanmpnenioot
€ tid 0 7 0 um fitared seawntor
т om!
® | =
IET гй
100 = ~ ‚з
[3 з вне к T7 B8 ® ee Ne мм
0 | ~~
|^ 0 s
=] eee a Cher nmphenloci
= = [80 pesas
= 9 F
2
E
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= №
n 1 ym їй ей azole
A, в 02 m {ЇЇг/ вй ssewaley
Unliitored ammwater
ту * фла M^ G HM uc
Pays linm spawieng
FIG.3, Effect of water treatment on growth and sur-
vival of Pecten fumatus larvae.
ә
uh
e
Tomarec Head, grew at mean rates of 2.8mm
week’ for sustained periods of up to 7 weeks at
20-23"C. This growth rate is higher than the
previously highest summer rates of 1.7mm week”
(Cropp,1985) for equivalent size P. fumatus
glued to tapes suspended fram midwater
longlines in Tasmania.
A trial to determine optimum stocking rates of
juvenile scallops reared in field upweller unit at
Tomaree Head was initiated on Christmas Eve
1992, About 20,000 spat averaging 5.6mm and
30тр were stocked at & different densities (5
replicates per density) into 40 miniature upweller
units, each of nested stacks of 8 interlocking
screens.
Under prevailing conditions, including mean
daily sea temperatures of 18-22°С and salinities
of 34-35g kg !, growth rate of spat of 6-10mm
shell height and 30-150mg live-weighl, was con-
strained by flow rates of below about 40ml р
biomass! min. For scallops in this size range,
suppression of growth due to crowding coincided
with a stocking density of about 0.3 g.cm" repre-
senting à surface area stocking rale approximat-
ing 100% of available screen arca.
Salinity tolerance of 1-2mm juvenile P.
Jfumatus was identified as a narrow range of 32—
38mg ml” outside of which significant mortality
occurs within 72h. These results were consistent
with salinity tolerances reported for adult P.
Jumatus (Nell & Gibbs,1986),
SPECIALIST HANDLING TECHNIQUES
Hypersaline baths (45 g.kg'') and exposure to
air (emersion) for 2 hours were effective in induc-
ing more than 95% of ] -3mm spat to detach from
nursery screens, Hypersaline baths created by the
addition of an artificial sea salt to seawater
produced greater spat detachment after 2h than
those created by equivalent additions of sodium
chloride, The rate of detachment in hypersaline:
baths was unaffected by increasing temperature
from 20-26°C, hut was depressed at | }°C. Addi-
tion of magnesium chloride (27g.kg!) to
Seawater and reduction of seawater pH 10 2 were
also effective in increasing spat detachment rate,
but not as effective as hypersaline baths or air
exposure. With the exception of spat exposed to
seawater containing !I5mg.kg' available
chlorine, no significant mortality and 9596 reat-
tachment occurred within 24h of all detachment
methods tested.
Of 14 compounds tested, only chloral hydrate,
Mg Сіз and Mg 504 induced anaesthesia in adult
MEMOIRS OF THE QUEENSLAND MUSEUM
scallops within 1h, Mg 504 was exeluded from
further testing due to high postanaesthesia mor-
tality. Doses of 4¢.1' Chloral hydrate at 4р1"
(0.024M) or MgCl» at 30g.I! (0.31M) were most
suitable on the basis of time to and recovery from
anaesthesia. Neither anaesthetic caused mortality
nor increased spawning activity, Mg Clo reduced
inadvertent spawning triggered by routine han-
dling and maintenance activities. Time to anaes-
thesia for both agents was found to be affected
(P«0.05) by waler temperature,
ACKNOWLEDGEMENTS
We thank staff of the BWFCRS for assistance
in manuscript preparation, Dr John Nell, Stephen
Battaglene and John Holliday for editorial com-
ments and Wayne Walker and John Kelly for
collection and delivery of broodstock from Jervis
Bay. This study was part of Fisheries Research
and Development Corporation grant 91/53,
LITERATURE CITED
CROPP, D.A. 1985, Scallops thrive in TFDA culture
program. Australian Fisheries 44(1): 16-18,
CROPP, D.A, & FRANKISH, K. 1989, Cost com-
parison of hatchery and natural scallop spat for the
scallop Pecten fumatus Reeve, Pp, 196-225, In
Dredge, M.L.C., Zacharine W.F, & Joll, L.M.
(eds), ‘Proceedings of the Australasian Scallop
Workshop, Taroona, Tasmania, July 1988" (Tas-
manian Government Printer; Hobart).
FRANKISH, K., GOARD, L. & O'CONNOR, W.
1990, Hatchery scallop culture success in NSW.
Australasian Aquaculture Magazine 4(9); 10-11.
FRANKISH, K., GOARD, L. & O'CONNOR, W.
1991. ‘The development of hatchery techniques
for the production of Sydney rock Oysters (Sac-
costrea commercialis), Internal Report, NSW
Fisheries, Brackish Water Fish Culture Research
Station, 28р,
FUENTES, HR, WILLIAMS, RJ, DIVER, L.P. &
SMITH, А.К. 1992. Commercial molluses of Jer-
vis Bay. Pp. 000-000. In ‘Jervis Bay marine
ecological study. Final Report’. (NSW Fisheries,
Fisheries Research Institute: Cronulla).
NELL, J.A. & GIBBS, P.J. 1986. Salinity tolerance and
absorption of L- methionine by some Australian
bivalve molluscs, Australian Journal of Marine
and Freshwater Research 37: 721-727.
SAMAIN, J.F., SEGUINEAU, C., COCHARD, J.C.,
DELAUNAY, F., NICHOLAS, J.L, MARTY,
Y., GALOIS, R., MATHIEU, M, & MOAL, J,
1992. What about growth variability for Pecten
maximus production? Oceanis 18; 49-66,
ASPERRIMUS (LAMARCK)
У.А, Q'CONNOR, М.Р. HEASMAN, A.W. FRAZER & J.J. TAYLOR
O'Connor, W.A., Heasman, M.P., Frazer, A.W, & Taylor, J.J, 1994 08 10: Hatchery rearing
the doughboy scallop, Chlamys (Mimachlamys) asperrimus (Lamarck). Memoirs of the
Queensland Museum 36(2): 357-360, Brisbane. ISSN 0079-8835.
Hatchery rearing and growout trials were conducted with the doughhoy scallop, Chlamys
(Mimachlamys) asperrimus (Lamarck) as a first step towards assessing their aquaculture
potential, In 1992, broodstock, from Jervis Bay were in peak reproductive condition in June,
August and September. Induced spawnings produced larvae thal took 18-20 days to reach
pediveliger stage and a further five days before all pediveligers had left the water column.
An estimated 10 000 settled spat were deployed on a longline in Port Stephens, grew to an
average 10mm and were transferred to lantern cages. Growth over ihe first year averaged
c.1mm per week and reproductive maturity was reached at 30-35mm shell height. Initial
observations suggest doughboy scallops have aquaculture potential and could be grown with
Pecten fumatus,
W.A O'Connor, M.P. Heasman, A.W. Frazer and J.J. Taylor, NSW Fisheries, Brackish Water
Fish Culture Research Station, Salamander Bay, New South Wales 2301; 9 March, 1994.
HATCHERY REARING THE DOUGHBOY SCALLOP, CHLAMYS (MIMACHLAMYS)
The doughboy scallop or fan shell, Chlamys
(Mimachlamys) asperrimus (Lamarck), is a sub-
tidal bivalve of southern Australia, found from
Shark Bay, Western Australia, to New South
Wales (Wells & Bryce,1988; Fig.1). Up to 1004-
mm long (Zacharin et а1.,1990), it is commonly
byssally attached to solid objects in depths of
7—69m (Young & Martin,1989). Unlike the com-
mercial scallop Pecten fumatus, it is unisexual
with the orange gonad of mature females clearly
distinct from the off-white gonad of males.
In the Pacific region Chlamys have provided
valuable fisheries butC. asperrimus has only been
uf minor commercial importance despite being
dredged in Tasmania (Sanders, 1970). In southern
Australia scallop fishing and aquacultural effort
are largely directed toward P. fumatus, although
increasing pressure on P. fumatus stocks could
see a revival of the past practise of fishing C.
asperrimus for sale as à ‘roe on’ product in the
same market (Young & Martin,1989). Alterna-
tively С, asperrimus'could potentially form а new
culture industry (Сгорр,1989). However, only
one report of its artificial propagation (Rose &
Dix,1984) has been made. More information on
its biology is required to evaluate its potential for
aquaculture and to allow management of
wildstocks should fishing effort increase.
METHODS
Broopstock
Fortnightly throughout 1992 scallop brood-
stock were collected by divers from Jervis Bay
(Fig.1); reproductive condition was determined
by macroscopic observations and through cal-
culation of the gonado-somatic index (GSI).
While stock capable of spawning were available
most of the year, the population was in peak
reproductive condition in June, August and Sep-
tember, This peak in condition is several months
later than reported for stocks in the D'Entre-
casteaux Channel, Tasmania (Grant,1971).
SPAWNING
Spawning procedures were initially based on
those of Gruffydd & Beaumont (1972). Brood
stock were scrubbed clean of fouling organisms
and maintained in the hatchery at the Brackish
Water Fish Culture Research Station
(BWFCRS), Port Stephens (Fig.1). Scallops were
-r ;
А "s
H5"s go iy 150
pn d C үз
, e n d
үт p ce y
| t
J | "
a au i^
j L
| Ü oT E
1 |
Shark Bay % + TETY.
`
new |
V уг 7 F Part Stephens
Geographe | т Sh of Jervis Bay
" Bay EA = $ му. "E
2 Chlamys asperrimus
FIG, i. Distribution of the doughboy scallop arqund
Australia.
358
A 3 | ү le
Shall WINN qm)
B 8
4
#
ы аы”
ACC
=
Ld — —
—— €— 4
5 £
Percent retention
0 a 8 wp б 29
Time (days)
= Spell width (um) « Percentretantion
5.0,
FIG.2. Growth and retention of hatchery reared C.
asperrimus laryae.
induced to spawn by placing broodstock in.a bath
of seawater (17.8°C, ambient to previous holding
tank) for 1h and then increasing temperature 3—
4°C over the following hour. Subsequent studies
have shown that mature individuals can also be
induced to spawn using intragonadal or intramus-
cular injections of serotonin, however, while a
greater proportion of females can be induced to
spawn with serotonin than with temperature in-
duction the average fecundity is markedly
reduced (O'Connor et al., unpubl. data).
RESULTS
LARVAL REARING
Larval rearing techniques and development fol-
lowed those of Rose & Dix (1984). When scal-
lops commenced spawning they were placed in
separate 5] beakers of seawater. Spawning in-
dividuals released 0.6х10° to 5x 105 eggs on many
occasions exceeding the maximum fecundity
reported by Rose & Dix (1984). As soon as pos-
sible following gamete release, sperm solution
from several animals was mixed and added to the
eggs. Fertilised eggs were then placed ina 10001
polyethylene tank at a density of 8 eggs ml”.
Trochophores (78,85, 1 шт width) were first
observed 24h after fertilisation and the first D
veligers (101+3.5рт in shell width) were ob-
served after 42h, 11% of the larvae sampled still
being trochophores at this time. After 48h ‘D’
veliger larvae were collected on nylon mesh
sieves and a 10001 tank stocked at 4 larvae/mI '.
Every 2-3 days larvae were sieved from the cul-
ture water and placed in à new tank of seawater.
Water lemperatures ranged 17.5-19.5°C during
the larval rearing period. Mean larval size was
MEMDIRS OF THE QUEENSLAND MUSEUM
determined daily, while larval densities were
determined at each water change (Fig.2), Larvae
were fed algae (Tahitian /sochrysis aff. galbana,
Pavlova lutheri and Chaetoceros calcitrans)
twice daily on an equal dry weight basis. Feed
rates were increased daily according to changes
in larval size and density, ranging from the equi-
valent of 3500-20000 T. Isochrysis cells larva!
day .
The first pediveligers were observed on day 18
after fertilisation and settlement substrates were
introduced on day 20. By day 25 larvae had left
the water column with the majority choosing to
settle on the base and lower wall of the tank,
SETTLEMENT
Four types of settlement substrate were placed
in the tank; PVC discs (140mm diameter);
monofilament mesh; 15mm black nylon mesh
and 5mm black nylon mesh bags. Four lines
supporting PVC discs were hung in the tank. Each
line supported 4 discs, equally spaced from the
bottom of the tank to the water surface. Three
bags each of the 3 types of mesh substrate were
weighted and lowered into the tank. After 3 days
the PVC dises were removed and settlement was
evaluated on upper and lower surfaces of each
disc. Settlement was poor (30—50 spat/disc) and
most spat settled in the central recess on the
underside of the disc. Mesh substrates were left
in the tank for a week before being deployed in
Port Stephens. Settlement on the mesh substrates
was not assessed until spat were large enough to
be retained by rhe surrounding bag if detachment
from the substrate occurred as a result of han-
dling. Following removal of all substrates, large
numbers of larvae were found to have settled on
the lower surfaces of the tank.
SPAT
Settlement on the mesh substrates deployed in
Port Stephens was assessed after 3 weeks. Spat
numbers were greatest upon black nylon mesh
collectors and spat were concentrated in regions
where mesh was densely packed,
The lack of C. asperrimus spat on other similar
materials held on the longline showed no natural
spat fall had occurred. Within twelve weeks spal
had reached 10mm in size and were transferred
to Japanese lantern cages. At this stage fewer than
6% of the number of the pediveligers put to sct
had been retained.
Growth in lantern cages in Port Stephens ap-
proximated Imm a week throughout the first year
(Fig.3) and sexual maturation occurred between
HATCHERY REARING THE DOUGHBOY SCALLOP
Shell height (mm)
8 в
b
N
\
\
o 10 20 30 40 55
Time (weeks)
— Satilement substrates = — lentem cages
F1G.3. Growth of hatchery reared C. asperrimus in
Port Stephens.
30-35mm shell height. Motile sperm was ex-
tracted from males of 26mm shell height, while
oogenesis was evident in females as small as
28mm. This may indicate precocious maturity in
the warmer northern extent of the species range
and warrants further investigation.
DISCUSSION
Experience in the hatchery production of mol-
lusc species at the BWFCRS has indicated that C.
asperrimus is well suited to mass production.
Larval and spat survival have been good, but
improved settlement techniques would be re-
quired. The recent success of nylon mesh screens
in downwelling systems to settle P. fumatus
(Heasman et al., this volume) larvae could be
extended to C. asperrimus as a means to exert
greater control of settlement. Similarly the poten-
tial for early maturation to retard growth needs to
be addressed. The possibility of growth retarda-
tion associated with the early onset of functional
maturity may be overcome in this species by
induction of triploidy.
The incidence of either parasitic trematodes
(Bucephalis sp.) or mudworm infestation
(Polydora sp.)in mature С, asperrimus, collected
from Jervis Bay, often exceeded 10 and 90%
respectively per collection. While Polydora sp.
has been a significant cause of mortality in P.
fumatus held in lantern cages in Tasmania (Dix,
1981), neither Polydora or Bucephalis have been
observed in C. asperrimus reared on longlines in
Port Stephens, Potential problems associated
359
with sale of wildstock from Jervis Bay, notably
mudworm, appear to have been eradicated with
suspended culture, although mudworm may be a
site specific problem.
Techniques used to rear C. asperrimus in these
preliminary trials have been closely based upon
those under development at the BWFCRS for the
commercial scallop, P. fumatus, and may not be
the most appropriate for this species. Adjust-
ments to larval rearing techniques, such as feed
rates and water temperatures, could improve lar-
val growth and survival, while different growout
techniques could benefit juvenile growth. Unlike
other commercially exploited Australian scallop
species, C. asperrimus retains the ability to form
bysall attachments throughout its life which may
permit the use of culture techniques developed
for similarly attached bivalves such as mussels.
ACKNOWLEDGEMENTS
We thank staff of the Brackish Water Fish
Culture Research Station for assistance, in par-
ticular Drs John Nell and Geoff Allan for com-
ment on the manuscript and Mr Lindsay Goard,
LITERATURE CITED
CROPP, D.A. 1989, Scallop culture in the Pacific
region. Pp. 134-153. In Dredge, M.C.L.,
Zacharin, W.F. & Joll, L.M. (eds), ‘Proceedings
of the Australian Scallop Workshop, Hobart, Tas-
mania, ]1988' (Tasmanian Government Printer:
Hobart).
DIX, T.G. 1981. Preliminary experiments in commer-
cial scallop (Pecten meridionalis) culture in Tas-
mania. Tasmanian Fisheries Research 23: 18.
GRANT, J.F. 1971. Scallop survey - D'Entrecastesux
Channel, Tasmania, 11-14 May 1971. Tasmanian
Fisheries Research 5(2):21.
GRUFFYDD, L.L.D. & BEAUMONT, A.R. 1972, A
method of rearing Pecten maximus larvae in the
laboratory. Marine Biology 15; 350-355,
ROSE, R.A. & DIX, T.G, 1984, Larval and juvenile
development of the doughboy scallop, Chlamys
(Chlamys) asperrimus (Lamarck) (Mollusca:Pec-
tinidae), Australian Journal of Marine and Fresh-
water Research 35: 315-323.
SANDERS, M,J, 1970. The Australian scallop industry,
Australian Fishing 29: 2-11.
WELLS, F.E. & BRYCE, C.W. 1988. ‘Seashells of
Western Australia’, (Western Australian
Museum: Perth),
YOUNG, Р.С, & MARTIN, R.B. 1989. The scallop
fisheries of Australia and their management,
Reviews of Aquatic Science 1(4); 615-638.
ZACHARIN, W., GREEN, R. & WATERWORTH, C,
1990. Estimated abundance of Doughboy, Queen
360
MEMOIRS OF THE QUEENSLAND MUSEUM
and Commercial Scallop stocks in the D’- cal Report 40, Marine Laboratories, Division of
Entrecasteaux Channel, Tasmania 1989. Techni- Sea Fisheries, Hobart, Tasmania.
TOXIC ALGAL BLOOMS:POTENTIAL HAZARDS TO SCALLOP CULTURE
AND FISHERIES
SANDRA E. SHUMWAY AND ALLAN D. CEMBELLA
Shumway, S.E. & Cembella, A.D. 1994 08 10: Toxic algal blooms: potential hazards to
scallop culture and fisheries. Memoirs of the Queensland Museum 36(2): 361—372. Brisbane.
ISSN 0079-8835,
Phycotoxins from algal blooms are accumulated by filter-feeding bivalve molluses. Since
only the adductor muscle of scallops has been traditionally marketed, scallops are not usually
included in routine monitoring programs but intensified aquaculture ventures in areas prone
to toxic blooms have provoked public health concerns, Our focus on the sequestering and
biotransformation of phycotoxins in scallops indicate that: 1) toxins are not distributed
evenly; toxin is usually concentrated in the mantle and digestive gland; 2) some scallop
tissues, e.g. digestive glands and mantles remain highly toxic throughout the year; 3) toxicity
varies (43.5%) between individuals in the same area; 4) no correlations could be made
between toxicity levels in gonadal and other tissues.
Scallop culture and commercial fisheries can thrive in areas prone to toxic algal blooms if
only the adductor muscle is utilized. Safe marketing of “roe-on” scallops is feasible only
under strict regulatory regimes. Marketing of mantles or whole scallops poses a high risk to
public health and should only be undertaken after extensive monitoring. Scallop maricul-
turists should be aware of risks associated with phycotoxins. Further, public health guidelines
with particular emphasis on toxin levels in individual tissues is necessary if scallops аге to
be marketed whole or in conjunction with tissues other than adductor muscles.
Sandra E. Shumway, Bigelow Laboratory far Ocean Sciences, West Boothbay Harbour,
Maine 04575, USA (present address; Natural Science Division, Southampton College LIU,
Southampton, New York 11968, USA)& Allan D. Cembella. Institute for Marine Biosciences,
National Research Council, 1411 Oxford Street, Halifax, Nova Scotia B3H 3Z1, Canada;
21 June 1994.
The impact of toxic algal blooms on scallop
culture and fisheries is often underestimated or
even ignored since traditionally only the large
adductor muscle is consumed. Adductor muscle
tissue is usually free of accumulated toxins of
algal origin (phycotoxins), although levels in ex-
cess of the regulatory limit may occur. Scallop
«ресс are, however, by no means exempt from
the effects of toxic algal blooms (Table 1),
Generally, scallops are not included in routine
monitoring programs for paralytic shellfish
toxins and they havc only recently been covered
by regulations of the Interstate Shellfish Sanita-
tion Conference (ISSC) in the US. Areas prone to
outbreaks of toxic algae overlap with areas where
scallops are fished or cultured commercially
(Fig.1). With expansion of scallop culture and
increased interest in marketing non-traditional
scallop tissues, as well as whole and ‘roe-on"
scallops, an understanding of the problems and
hazards posed by toxic algae to the scallop in-
dustry is required,
Scallops are common inhabitants of nearly
every coastal region worldwide and support
major commercial fisheries and mariculture in-
dustnes (Hardy,1991; Shumway,1991; Fig.1).
Blooms of toxic and noxious algae are also
regular cosmopolitan events (LoCicero,1975;
Taylor & Seliger,1979; Anderson et al.,1985;
Okaichi et al.,1989; Granéli et al.,1990; Hal-
legraeff,1993). Their impact on utilization of
shellfish resources was reviewed by Shumway
(1989, 1990) and Hallegraff (1993). Filter-feed-
ing bivalves, such às scallops, accumulate toxic
algac and associated toxins in their tissues render-
ing them vectors of various types of seafood
poisoning, including paralytic shellfish poison-
ing (PSP), diarrhetic shellfish poisoning (DSP),
amnesic shellfish poisoning (ASP). Such
shellfish are unfit for human consumption.
While some groups of toxic algae have had
devastating effects on scallop populations (Table
1) the primary threat to industry and public
health is the potential for human illnesses such as
DSP and PSP. Scallop culture and fisheries have
been conducted in areas prone to blooms of high-
ly toxic algae, c.g. Canada, Japan, United States,
France (Shumway, 1990,1991; Fig).
Since only the adductor muscle is generally
consumed in North America, scallops are usually
362
ve
7
A Argopecten
Argopecten
MEMOIRS OF THE QUEENSLAND MUSEUM
Amusium
ЕІС 1. Geographical distribution of PSP, DSP, NSP and ASP toxins and their relationship to commercially utilised
(fished and cultured) scallop genera.
shucked at sea where shells and unwanted tissues
are discarded. These ‘other’ tissues include the
mantle (rims or rings), gonad (roe), digestive
gland (hepatopancreas, liver), and gills; together
they comprise over 80% of the total weight of
tissue (Schick et al.,1992; Fig.2). In some areas,
e.g. Europe, Japan and southem Australia, scal-
lops are sold with the gonad attached (‘roe-on’)
and there has been a steady and continuing inter-
est in fuller utilization of scallop tissues in other
regions, particularly the US and Canada (Bourne
& Read,1965; Dewar et al.,1971), The idea that
consumption of scallops is always safe should not
be accepted unreservedly. While scallops are not
the most common vectors of paralytic or diar-
rhetic shellfish poisonings, there have been
several reported illnesses and even some deaths
attributed to toxic scallops.
Prior to the occurrence of PSP toxins on Geor-
ges Banks, Bourne & Read (1965) advocated the
marketing of scallop muscles with attached roes
and rims (mantles). Dewar et al. (1971) presented
procedures and recipes for production of high-
quality frozen and canned products and, based on
results of Japanese taste panels, indicated con-
sumer acceptance of these products. The sea scal-
lop, Placopecten magellanicus, and Japanese
scallop, Patinopecten yessoensis, support the two
largest scallop fisheries worldwide; P. magel-
lanicus is the focus of efforts to market roe-on
product and whole animals, as is done with Pr.
yessoensis. There is a renewed interest in market-
ing both whole and ‘roe-on’ scallops (P. magel-
lanicus) from Canada and the northeastern US
(Gillis et al.,1991; Merrill, 1992), and whole pink
scallops (Chlamys rubida) from the Pacific
northwest (Nishitani & Chew,1988). However,
the first reported incidence of PSP toxins on
Georges Banks (Sharifzadeh et al.,1991; White et
al.,1992a) and the persistent presence of these
toxins in the Pacific northwest (Nishitani &
TOXIC ALGAL BLOOMS AND SCALLOP CULTURE
Chew,1988; Shumwav,1990) has sparked new
concerns with regard to consumer safety,
Scallops arc opportunistic filter feeders which
utilize both pelagic and benthic microorganisms
as food sources (Shumway et al.,1987; Bricelj &
Shumway, 1991). These organisms are consumed
and concentrated in the digestive gland. Where
toxic algal species are present, the shellfish be-
come vectors of shellfish poisons including PSP
and DSP. Poisonings due to PSP have been
reported after consumption of both sea scallops
(Medcof etal..1947; Washington Office of Public
Health Laboratories and Epidemiology,1978),
and pink and spiny scallops consumed whole
(Canadian Department of Fisheries and
Oceans, 1989), Seafood poisoning attributed to
DSP following consumption of scallops has been
known in Japan since 1977, and has resulted in
several hundred illnesses (Nomata, pers. comm.).
On September 23, 1983, a 5 year old boy died of
PSP after eating scallops from Olotayan Island in
the Philippines (Estudillo & Gonzales,1984).
One death was reported from consumption of
Chlamys nipponensis in Iwate prefecture, Japan
in 1962 (Nomata pers. comm.) and a death was
also attributed to consumption of Hinnetes in
California (Sharpe, 1981).
Accumulation of PSP and DSP toxins has al-
ready had devastating effects on the scallop in-
dustry (both cultured and fished), especially in
areas such as the Atlantic and Pacific coasts of
North America and in Japan where toxic blooms
arc regular events (Ogata ct al.,1982; Gillis et al.,
1991; Nishihama,1980), Japan has stopped sup-
prine whole scallops to large markets such as
"rance because of the presence of PSP toxins
(Merrill, 1992). Careful monitoring of 'roe-on'
scallops in Canadian waters resulted in closure of
*roe-on’ fishing for most of the Canadian sector
nf Georges Bank during 1989 and 1990, when
PSP toxin levels exceeded the tolerance limit
(80g STXeq/100g) (Gillis et al.,1991). Efforts
are currently underway by the National Marine
Fisheries Service (NMFS) to develop a protocol
for certification of *roe-on' or whole scallops
(Placopecten or Argopecten} harvested in US
federal waters west of 71°W longitude,
Problems associated with scallop toxicity
monitoring are exacerbated by high variability in
toxicity between individual animals ( Whitefleet-
Smith et 31,1985; Gillis et 31,1991; White et
a1.,1992h). This vanability can be considerable
(Beitler, 1991; White et al., 19926 and references
therein) and has been attributed to differences in
season, geographical location, specific toxins in-
363
FIG.2. Diagrammatic representation of scallop tissues,
volved and toxin concentrations, Finally, biocon-
version of toxins berween/within scallop tissues
may also account for some of the variation in
toxicity,
Total toxicity varies not only between locations
and individuals, but also among tissues of in-
dividual scallops. Most available data on PSP
toxin distribution among scallop tissues are for
the sea scallop, P. magellanicus and Japanese
scallop, Pt. yessoensis (Table 2), The digestive
gland (hepatopancreas, liver) is usually the most
toxic tissue; with levels in excess of 45,000p2
STXeg/lO0g, as determined by AOAC mouse
bioassay, having been recorded in the Gulf of
Maine (Watson-Wright et al,,1989). This is of
particular importance for marketing of whole
scallops and special care must be taken in any
area where toxic algae are present to ensure
regular testing for these toxins.
High levels of PSP toxicity have also been
reported for gonadal tissue (roe), Watson- Wright
etal, (1989) reported detectable PSP toxicity (Le.
40ug STXeq/100g) in 69% of scallop gonadal
samples analyzed (n241) from the Bay of Fundy,
hut found no correlation between the toxicity of
gonadal tissue and other tissues. While these high
ioxicity levels (e.g 1300p gSTXeq/100g tissue in
P. magellanicus from Mascarene, New
Brunswick; Microbiology Division, Food Re-
search Laboratorics Health and Welfare, Canada;
Black's Harbour, New Brunswick data reports),
seem to be the exception rather than the rule, they
again point to the need for strict monitoring prac-
364 MEMOIRS OF THE QUEENSLAND MUSEUM
TABLE 1. A summary of toxic and noxious мая blooms associated with ieee oe
affected
Chlamys nipponensis Anraku (1984)
Patinopecten yessoensis
Pecten albicans
highly toxic; not | Northumberland | Ayres & Cullum (1978);Ingham et al.
U.K. (1968); Wood & Mason (1968)
Alexandrium
lamarense
Chlamys opercularis
Pecien maximus
Gulf of Maine
and eastern
Canada;Bay of
Fundy and
St.Lawrence
regions
Prakash(1963); Bourne (1965); Caddy &
Chandler (1968); Prakash et al. (1971);
Medcof(1972); Hurst (1975); Hartwell
(1975); Hsu et al, (1978); Tufts (1979);
Jamieson & Chandler (1983); Shumway
et al. (1988); Gillis et al. (1991); Cembella
& Shumway(1991)
| тпар gellanicus Gulf of Maine а;
Е Patinopecten yessoensis
Bicknell & Collins (1973
h tamarense Райлоре ten yessoensis |toxic [Japan [Seki uchi et al. (1989)
А. tamarense |Placopecten violent swim- laboratory Shumway & Cucci (1987); Gainey &
magellanicus ming aclivity; Shumway (1988a,b)
А. tamarense |Placopecten
magellanicus
highly toxic
production of
mucus
Lassus et al. (1989)
n ттт гта шш —] Mariyama et al. (1983)
C. nobilis
[Alexandrium |Patimopectenyessoensis [toxic [эрт — (Nishihama (1980)
A.catenella — | Patinopecten yessoensis
C. nipponensis тайашга
Chlamys hastata illness
гай viscera
[4. catenella — | Hinnites multirugosus — |toxic Pacific USA Nishitani & Chew (1988)
| Chlamys hastata,Pecten
caurinus,'Pecten sp.
Chlamys patagonicus Avaria (1979);Guzman & Campodonico
(1978)
Gymnodinium | Equichlamys bifrons, toxic Tasmania Hallegraeff & Summer (1986);
catenatum Mimachlamys Hallegraeft et al. (1989); Oshima et al.
asperrimus, Pecten (1982, 1987a,b)
Noguchi et al. (1978, 1980a,b, 1984)
first report of
toxicity by this
Japan Ikeda et al. (1989)
scallop deaths;
recruitment
failure |
North Carolina | Barris (1988); Tester & Fowler (1990);
Summerson & Peterson (1990)
TOXIC ALGAL BLOOMS AND SCALLOP CULTURE
TABLE 1 (continued)
|G. veneficum — | veneficum
Gyrodinium
aureolum
| Gv. aureolum
Pecten maximus
Gy. cf. aureolum — | Pecten maximus
[cesi 7 Placopecten
magellanicus
anophagefferens irradians
А. anophagefferens |Argapecten
O2 depletion
mass mortalities
А. anophagefferens | Argopecten
of year class
Rhizosalenia chunii | Pecten alba
[not specified — | Chlamys nobilis toxic
Alexandrium tamarense (=Gonyaulax tamarensis =Protogonyaulax tamarensis); A. catenella (=Gonyaulax
Algal spe Bpecies "| Shellfish species моз) = Location | References
affected
Abbott & Ballantine (1957
mortalities in young scallops
Pecien maximus numbers of larvae declining |LoughHyne, — | Minchin (1984)
during bloom Ireland
high mortality in post-larvae
and juveniles; reproduction
and growth inhihited in adults
nontoxic; mortalities due to
larval shell growth reduced
and mortalities increased
76% reduction m adductor
weights; recruitment failure
bitter taste rendered shellfish
unmarketable for 7 months;
digestive gland lesions and
shellfish mortalities
not specified, prob- | Patino. yessoensis, | toxic
ably Alexandrium — | Chlamys farreri
365
ке Lassus & Berthome (1988)
France Erard-LeDenn etal. (1990)
New York Mahoney & Steimle (1979)
Bight
laboratory Gallagher et al. (1988)
Long Is, NY;
Narragansett
Bay, RI;
Barnegat Bay,
NI
Long Island,
NY
Cosper et al. (1987); Tracey
et al. (1988); Tracey (1985);
Smayda & Fofonof (1989)
Bricelj et al. (1987)
Australia
. |Japan.
Parry et al. (1989)
Jeon et al. (1988)
Nagashima et al. (1988)
catenella -Protogonyaulax catenellay, Gymnodinium breve (=Piychodiscus brevis)
tices if scallop products other than adductor
muscles are to be utilized.
ADDUCTOR MUSCLE TOXICITY
It had been generally accepted that scallop ad-
ductor muscle tissue tends to remain free of ac-
cumulated toxins (Medcof et а1.,1947; Watson-
Wright et al.,1989; Shumway,1990); however,
reports of adductor muscles with measurable tox-
icity, and even scores exceeding the regulatory
limit of 80ug STXeq/100g tissue have been
reported for several scallop species (Table 1). It
is impossible to estimate the toxicity of adductor
muscles of scallops based on the toxicity of sur-
rounding visceral tissue (Beitler, 1991; Watson-
Wright et al.,1989) and no assumptions regarding
the toxicity of individual scallop tissues should
be made on any such correlations.
DETOXIFICATION
In addition to individual variations in toxin
levels and among tissues within individuals, scal-
lops exhibit slow and markedly variable rates of
detoxification. All data available are for PSP
toxins and are limited to Pt. yessoensis, P. magel-
lanicus, C. nipponensis akazara and Pecten max-
imus. Once PSP toxins are accumulated by
scallops, they are only slowly eliminated. Detec-
table PSP toxicity (40.2 STXeg/100g) by AOAC
mouse bioassay has been reported to persist in Р,
magellanicus for extended periods ranging from
several months to two years (Medcof et al., 1947;
Jamieson & Chandler,1983; Shumway et al.,
1988; unpubl. data). Digestive glands of Pt. yes-
soensis (initial toxicity 34,0004gSTX eq/100g;
ie. 1700MU/g tissue; MUzmouse units) were
reported to contain 2,0004gSTX eq/100g
(100MU/g) even after being held in running
366 MEMOIRS OF THE QUEENSLAND MUSEUM
TABLE 2. Levels of paralytic shellfish toxins ( р, STXeq/100g tissue) recorded in scallop species from various
geographical locations. A conversion factor of 1 MU =~ 0.20 g STXeq has been used to standardize data sets.
SPECIES TISSUE сехи LEVEL LOCATION REFERENCE
g STXeg/100g)
Chlamys Ofunato Bay, Japan Noguchi et al. (1978)
nipponensis
akazura
[7 ercularis es T
Chlamys rubida | Adductor 6 [Washing eton, USA Anonymous (1987)
Adductor muscle British Columbia DFO (1989)
Viscera+
Crassadoma
gigantea”
Washington, USA
California, USA Anonymous (1980);
Sharpe (1981)
Patinopecten Adductor Alaska, USA Anonymous (1987)
caurinus
Patinopecten Adductor Ofunato Bay, Japan Noguchi et al. (1978)
yessoensis Совету?
Funka Вау, Japan Noguchi et al. (1980a,b)
Digestive gland
Other
Patinopecten Ofunato Bay, Japan Sekiguchi et al. (1989
yessoensis Ofunato Bay, Japan | Kodama et al. (1990
Kawauchi Bay, Japan _| Ogata et al. (1982
Digestive gland 15000 Funka Bay, Japan Nishihama (1980)
Digestive gland 130,000-220.000 Japan Noguchi et al. (1984)
Adductor muscle 60-260
Hepatopancreas 34,000 ~ lOfunato Bay, Japan Oshima et al. (1982)
Digestive gland 42,000-70,000 Ofunato Bay, Japan Maruyama et al. (1983)
Rectum 4200-12,400
Foot 3200-4600
Gonad 2200-3200
Mantle 1500-2200
Gill 1420-2200
Adductor muscle 320-860"
Digestive gland 15,000 Funka Bay, Japan Nishihama (1980)
Pecten maximus 1568 Farne Bank, UK Ingham et al. (1968)
Whole(?) 2700 laborato Lassus et al. (1989)
Pecten grandis — Whole 1520 Lepreau Basin, New Medcof et al. (1947)
(=Placopecten Digestive gland 8000 Brunswick
magellanicus) Gill 560
Adductor muscle <40
Gonad 190
Other 680
TOXIC ALGAL BLOOMS AND SCALLOP CULTURE 367
TABLE 2. (continued)
g STXeg/100g)
Gonad 44
magellanicus
Adductor muscle Bay of Fundy, Canada |Hsu et al. (1979)
Gonad
Hepatopancreas
Gill
Rims
Placopecten Adductor Maine, USA Shumway et al. (1988;
magellanicus — | Gonad unpubl. data)
Digestive gland
Placopecten Georges Bank (Loran White et al. (1992a,b);
magellanicus 1336543777) Shumway (unpubl)
Whole (— adductor)
Placopecten Hepatopancreas Вау of Fundy, Canada — |WatsonWright (1989)
magellanicus |Gonad
Adductor muscle
Gills
Rims
Whole
Digestive gland
Gonad
Adductor
Gill
All other tissues 1983)
Mascarene, Nova Jamieson & Chandler
he estive gland == 000 Scotia, Canada
дее? = sd 36-66 N Edge, Georges Bank
Gonad
Liver
Mantle
Adductor
Gill
Placopecten
magellanicus
Bay of Fundy, Digby, Jamieson & Chandler
Canada (1983)
Placopecten
magellanicus
* maximum reported values
+ whole body minus adductor
* probably leached from other tissues; scallops were frozen whole for
several months prior to dissection and analysis
stomach and digestive diverticulum
seawater for five months in the laboratory demonstrated that canning might be applicable
(Oshima et al.,1982). for scallops with PSP toxicity at levels as high as
Cooking can reduce toxin levels considerably 8,000n2STXeq/100g (400MU/g) tissue.
(Medcof et al.,1947; McFarren et al.,1960) and Freezing does not appreciably reduce toxin
canning has been used to reduce toxicity of scal- levels, although long-term storage at tempera-
lop tissue to acceptable levels (Noguchi et tures from 0 to -20°С may lead to some degrada-
al.,1980a,b); however, as a means of reducing tion of specific toxins, often to more toxic
toxicity, canning is usually only effective when derivatives. Moreover, freezing of whole scallops
toxin levels are relatively low, although Noguchi can result in migration of toxins from highly toxic
368
tissues, e.g. digestive gland, into adductor
muscle, rendering the latter unsafe for human
consumption (Noguchi et al., 1984; unpubl, data),
Toxin can also leach from attached gonads to the
adductor muscle during shipping (Bruce &
Delaney,1972).
PRECAUTIONS
A marke! for roe-on scallops 15 feasible only
under strict monitoring for algal toxins. Estab-
lishment of public health safety guidelines with
particular emphasis on toxin levels in individual
body parts is a necessity if scallops are 10 be
marketed whole or in conjunction with any tis-
sues other than adductor muscles.
Marketing of rims (mantles) nr whole scallops
can pose a high risk public health and should only
be undertaken under the strictest of monitoring
plans, The economic success of such an industry
is questionable.
Mariculturists should be acutely aware of the
potential risks and dangers associated with toxic
algal blooms and the marketing of various scallop
products.
Successful culture facilities and commercial
fisheries can persist in areas prone to toxic algal
blooms; however, only through careful site selec-
tion and monitoring can optimal utilization of
scallop resources be realized and economic losses
kept lo à minimum.
ACKNOWLEDGEMENTS
This project was funded by 3 grant cooperative
agreement (4NA-90-AA-HSK030) from the Na-
tional Oceanic and Atmospheric Administration
(NOAA) awarded to the New England Fisheries
Development Association, The views expressed
are those of the authors and do not necessarily
reflect the views of NOAA or any of its subagen-
cies, nor those of any Australian fisheries ad-
ministration agency,
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DILEMMA OF THE BOUTIQUE QUEENSLAND SCALLOP
B. HART
Hart, B. 1994 08 10: Dilemma of the boutique Queensland Scallop. Memoirs of the
Queensland Museum 36(2); 373-376, Brisbane, ISSN 0079-8835.
With increased production of the saucer scallop (Amusium balloti) in Western Australia and
Queensland over the past three years, great pressure has been placed on the Queensland
scallop to maintain its share of two niche markets in the face of forces which are changing
market conditions. Western Australian scallops have been sold in Singapore and Hong Kong
beneath Queensland prices. Many buyers are now finding Western Australian scallop
acceplable ‘at the price’. To redress this situation and diversify into other markets, the
Queensland scallop industry must be more price competitive.
B. Hart, B. Hart Enterprises, 444 Queen Street, Brisbane, Queensland 4000; 9 March, 1994.
The market for Australian saucer scallops
(Amusium ballot) is unusually specialised. A
niche market for roc-off saucer scallop meat com-
mands a substantial price premium in Singapore
and Hong Kong, Queensland and WA saucer
scallops are closely related but not identical and
have not been interchangeable in a market sense.
In this paper, I describe markets for saucer
scallop meats. Costs associated with perishable
commodities never decrease — there are unavoid-
able costs associated with interest, insurance,
cold store charges and the like, Penshable com-
modity markets can be volatile. The main method
of reducing risk is to sell as produced, providing
cost was covered and hopefully a small profit
margin maintained, If, on the other hand, market-
ing does not equate with product cost, withdrawal
from that particular item, al least on a temporary
basis, is indicated.
THE QUEENSLAND SCALLOP FISHERY
In 1978, QLD scallop meat exported was 200—
300tonnes. The fishery was regarded as a short
term ‘fill-in’ between prawn seasons. On a quan-
tity - price basis, prawns were regarded as more
attractive to fishermen. Since 1978 there has been
a steady build-up of trawlers which now regard
scallops as their main target. These are mainly
smaller short-range vessels which lack special-
iscd refrigeration. Large trawlers from far north
Queensland still travel to scallop grounds off
Hervey Bay, Bundaberg and Gladstone at times
of the year when prawn catches are low or
seasonal prawn closures are in place. These boats
may work on scallops for 3-4 months.
Scallop catches usually increase from August,
With peak catches in October-November; they
decline from January to May-June. These peaks
coincide well with the period of increased pur-
chases in Hong Kong and Singapore, leading up
to Christmas and the Chinese New Year.
The main management commitment for QLD
Mery is a size limit of 95mm between May 1
and November 1 to reduce fishing and ensure
adequate breeding scallop for spat fall the next
year, Otherwise the size limit is 90mm. Shell size,
net length and mesh size, and a ban on daylight
trawling, ensure adequate management so far as
marketing is concerned.
MEAT SIZE AND COUNTS
The market for roe-off scallop in SE Asia is
siructured in relation to meat size, Meat size has
been expressed 1n count per pound, with 3 clas-
ses: 20-30 to the pound, 20—40 to the pound, and
41-60 to the pound. Larger meats (lower counts)
attract a higher price, with the differential be-
tween top and second count meat 10-20% (Table
1). There has been discussion of introducing a
minimum shell size of 95mm on a year round
basis, to reduce the proportion of 41—60 scallop
meat on the market. This size scallop 1s important
in Overseas sales, as the W.A. fishery produces
little 41—60 count meat, and then normally at the
end of their season. The QLD fishery is thus able
to fill this gap. From a marketing perspective
TABLE 1. Scallop meat count from one processor's
records (September 1992 - March 1993),
Meat size-count per pound — | Proportion of landings %)
20/30 22
20/40 43
374
TABLE 2, Summary of scallop exports and prices
from Australia, (*=re-export)
Value (A$/kg)
QLD exports (tonnes)
Value (AS/kg)
Total exports
(Tonnes)
E J
вам [им [з
ЕА ае 8
| 1990 |
—— 20—128 at san тзт
eas ua
ingapore| 1989 | 137 | 43 |$19.30| 921 pa
|_| 1990 | 192 | 6s |$15.70| 123 [$20.70]
СЕСЕ $14.65 a3 [s2001]
L— me s [2e [ss] э jme
Taiwan
Lone у шз ||
amr UNE $12.61
Pam
$11.73
ажо | 1 | 70 [ео |_|
mime | Dp I
аю is] || ]
Serer Ганга n
L— [meme meson] 1 |
кеев | | [| [Г]
[nee] Гг г Г
nothing should be done to interfere with the
natural rin of size beyond size limits in force,
EXPORT DESTINATIONS
No information relevant to the quantity of scal-
lop imported into Singapore was available from
Austrade, but the following comment on export
to Hong Kong is informative: ‘Unfortunately
there are no disintegrated statistics published by
any local source, official or private, on imports of
scallops into Hong Kong. Import figures covering
this product are incorporated, with those relating
MEMOIRS OF THE QUEENSLAND MUSEUM
to all sorts of clams, mussels and other shell fish,
under a composite category “Molluscs other than
Cuttlefish, Squid and Octopus”. We have dis-
cussed with a number of major seafood importers
/ distributors, all of whom unanimously agreed
that Australia is presently the largest supplier of
frozen scallops in this market, accounting for
about 70% of the overall sales. Canada is the
second major source, sharing, however, no more
than 15% of the market. The balance is split
between the U.S.A. and Japan. A number of
major importers believe that total imports in 1992
could be 2,000-2,500t, of which 1,400-1,800t
were from Australia, Consumption by the market
in 1992 increased over 1991, probably by c. 10%.
Re-exports are reported to be insignificant, al-
most all being made to Macau and Guadong,
southern China. The trade estimated re-exports
averaging less than 5% of total imports,”
The Bureau of Statistics - Foreign Trade Inter-
rogation Facility - supplied data on Australian
scallop exports to major importing nations (Table
2). Apart from the 7 major destinations, minor
export tonnages were made to 22 countries. The
most significant were to Japan, Korea and
Malaysia, and originated from Western Australia
and Victoria. The main shipment to Japan was 20t
in 1990, at the very good price of A$43.83 FOB
per kg. The main source of scallop exported to
Malaysia has been Western Australia (56t, price
average A$12.37) and, surprisingly, South
Australia (16t at A$10.73). Australia imports a
quantity of scallop in various forms, including
frozen, dried, salted and brined (Table 3).
PRICE STRUCTURES
F.O.B. Brisbane prices of 20-40 count OLD
scallops (median size) from February—early
March shipments are compared with import
prices (Table 4). If the 1993 US$14.00 was con-
verted at the .8205 rate of March 1989, the FOB
value would have been A$16.63 per kg rather
than A$19.62 attained, The declining A$ has
helped maintain prices in the Australian fishery,
There is little scope for price comparison be-
tween QLD and imported scallops, much of
which is either breaded or imported for breading.
The smallest meats (41-60 pieces per pound)
produced in the QLD fishery are used only in
times of slow sales for local marketing and bread-
ing for the local trade. During 1992, W.A. scal-
lops were purchased by QLD wholesalers for the
restaurant trade and general distribution, due to
excessive prices of the QLD product.
DILEMMA OF THE BOUTIQUE QUEENSLAND SCALLOP
TABLE 3, Мае риу of кар into Australia
Ara са
3 nation (t supplied)
[1989] 1001 | P XE 356) А%11.68
арап (606) A$13.25
A$13.19
[1990] 1527 |
1991| 7065 |
Lir] 447 | Apos) | asua |
SUPPLY AND DEMAND
Production increases in W.A. and Queensland
present a real dilemma to the QLD scallop fishery
in the past three seasons. Annual production
(Table 5) shows that the W. A. fishery has been
well above average, exceeding QLD production.
Market prices (Table 2) indicate that scallop
connoisseurs of the world are 1n Hong Kong and
to a lesser extent in Singapore, where especially
attractive prices have been paid for the QLD
product. This is attributed to the good quality,
colour and texture which has earned premiums of
US$6.00 per kg and more on occasions, above the
same size W.A. product.
The Hong Kong market is of the order of 2,5001
per year, with about 70-75% from Australia.
Virtually no roe-on scallop is consumed in Hong
Kong, but some is used in Singa and Taiwan.
If the Australian proportion of the Hong Kong
market is assumed to be approximately 1,800t
annually and the equivalent Singapore demand is
6001, the production position and distribution for
1990-1992 appears as in Table 6,
Total W.A. and QLD production probably of-
fered a full supply in 1990 to the existing market
demand for Australian product. In subsequent
years production exceeded the quantity that Hong
Kong plus Singapore could absorb. W.A. is sup-
plying many more markets than Queensland (in-
cluding the U.S.A., Taiwan, France and the U.K.)
Wholesalers from these destinations baulk at
paying the premium price paid for QLD scallops
which has been available in Hong Kong and
Singapore; thus QLD wholesalers are left with
TABLE 4. FOB prices of 20—40 count scallop meats
for February-early March shipments of Queensland
saucer scallops.
[ — [Price (US$) | FOB Price (AS) | Exchange Rate
(1089 | $1400 | $19.62 m
E $17.00 $21.97
[1901 | $18.00
[152] ыз | $25.20 7646
375
only two viable markets. During 1992, many
Hong Kong buyers expressed the view that the
high price premium of QLD scallop over W. A.
scallop should be reduced, not because W, A.
scallop meat quality had improved, but because
many restaurants and other users were happy with
the W.A. any, ‘AT THE PRICE’. This has a
most serious implication for QLD scallop (Table
7). These data illustrate Queensland's declining
share in its two niche markets: so much so that
Hong Kong buyers are advising the QLD scallop
share of their market is about 30% (Table 7).
As Queensland again experienced very strong
production during 1992 (approximately 2,0001)
and excellent landings during January and
February 1993, the conclusion that substantial
unsold stocks of scallops exist in Queensland
must be drawn.
TABLE 5. Annual production of saucer scallops in
Australia.
b —— 1 WA (tonnes) QLD (tonnes)
792
1980 745
[5e i w 1539
э [эз |] — m |
In the period between April and күнү:
sales of scallops in Hong Kong and Singapore
will be reduced. The entertainment and festive
accasions which is the period of highest demand
start again in about September. There are two
clouds on the honzon for QLD scallops: 1, the
large stock of scallop stored and 2, an anticipated
good season for the W.A. fishery.
DISCUSSION
Boutique has been interpreted as being of spe-
cial quality and attraction in а niche market situa-
tion. If this reasoning is correct, the markets
which have been willing to pay a premium for
QLD scallops are limited by that premium over
other acceptable scallop qualities. The quality of
W.A. scallop has been demonstrated to he accept-
able ‘at the price’ and this has resulted in substan-
tial crosion of Queensland's market share in the
only markets willing to pay а premium for
Queensland quality.
The dilemrna arises as a consequence of strong
production rises 1n both Australian producers of
saucer scallops in much the same market period.
How does Queensland defend its market share in
176
TABLE 6. Production, exports and imports of saucer
scallops
Tota | W.A. | QLD
produxci- | product- | product-
ion (Ñ jon ion
Hong
Kong
imports
| 2005 | ass | 1539 | 1800 |
Под] | 3as2 | 2532 | s20 | таю | юю |
Singa-
pore
imports
its niche markets? Quality of handling and pack-
ing must be maintained at all times, but if higher
production, in the order of the last few vears, is
maintained, Queensland must address the supply
and demand effects of market forces by making
strong attempts to diversify to other destinations.
Market diversification by QLD wholesalers at
prices similar to, orat a slight premium aver W.A.
scallop js possible. One Hong Kong buyer has
indicated that he would return to purchasing QLD
scallop if the premium was reduced to about
US$1.50-2.00/kg. Should this occur, we could
expect rationalisation in the catching sector,
which could haye marketing implications.
Ii the last 4 years, QLD scallop exports have
been almost exclusively to Hong Kong and Sin-
gapure. W. A. sales, on the other hand, have been
to 3 major markets, and to 5 other substantial
markets in which QLD wholesalers do not par-
ticipate.
Production and exports for 1992 (Table B) in-
dicate quantities of scallops from W.A. and QLD
which have been sold locally or held in store.
There is no way of estimating the local sale
component of the 1250t which appear not to have
been exported at the end of 1992, nor of the
quantity shipped in early 1993, but whatever this
amount may be there is still a substantial stock
awailing sale and export, Much of this stock will
attract costs associated with cold storage, which
cannot be recovered on the basis of current
market prices from Hong Kong or Singapore.
Stock purchased at lower prices subsequent to
this time can still be profitable at current prices
from Hong Kong and Singapore, but could not
cover bare costs at prices in other markets.
TABLE 7. Market share of saucer scallops.
wa] фр wa] о
| 1990) 162 вж] 35*| 65%]
| a| xm| 6% 809 20%
MEMOIRS OF THE QUEENSLAND MUSEUM
All of this presupposes contiouing high produc-
tion, which may be influenced by weather factors,
rain, water temperature, currents and other vari-
ables. As if these variables were not enough to
contend with, the exchange rate has had an ad-
verse effect on prices. The A$ strengthened
against the U,S.$ from .6785 to .7157 (bank to
buy) at the end of March 1993. For one particular
grade of scallop we sold at US$14,31 candf Hong
Kong this reduced the A$ value by A$1.09/kg.
Marketers of QLD scallop will continue to seek
out and sell in the best world destinations. If these
are restricted by price to two markets and market
circumstances do not change, the present product
in store may not be marketed until the end of
1993_ Quite obviously, costs of product in store
can never be reduced.
ТАВІЕ $. Production and export destination of saucer
svallops in 1992.
destination roduction (t
| Но Ко | 1004 |
Estimated unsold stock 1076
and local sales
Total production _ 4144 |
Comments from an overseas buyer who sup-
plied pricing information to us include “Unfor-
tunately most of us in the seafood commodity
business operate on the basis of an infinite num-
ber of short run decisions and cannot afford (or
believe we cannot afford) the luxury of long-
range planning. The Queensland fisherman and
99.9% of the seafood industry, does not under-
stand the problem if we lose our niche market, We
allow the serene song of the highest beach price
to destroy what maximises revenue over the long
run. We believe the short run is the long run and
are not prepared for Jogical results of our illogical
assumption, If the W.A. scallop destroys
Queensland's niche market, the long run revenue
implications for the Queensland scallop are not
promising.
In many Ways the decade-long 80's bubble of
Japanese stocks and properties poured gasoline
onto the fire of supply driven seafood markets.
We are now being forced to come to terms with
the charred remains and it is not à joyful èx-
perience.”
CONTENTS
ZACHARIN, W.
Scallop fisheries in southern Australia: managing for stock recovery ....... ПЕМ АНААН У: 241
FUENTES, H.R.
Population biology of the commercial scallop (Pecten fumatus) in Jervis Bay, М$\ЧУ/................ 247
JOLL,L.M.
Unusually high recruitment in the Shark Bay saucer scallop (Amusium ballori) fishery .............. 261
CROPP, D.A.
Hatchery production of Western Australian scallops ........... eeclesie 269
DREDGE, M.C.L.
Modelling management measures in the Queensland scallop Ёіѕһегу............................. 277
SHUMWAY, S.E. & CASTAGNA, M.
Scallop fisheries, culture and enhancement in the United States ................................ 283
ZACHARIN, W.
Reproduction and recruitment in the doughboy scallop (Chlamys asperrimus)
in the D'Entrecasteaux Channel, Tasmania. ............ 00s cee cece een nn 299
MCLOUGHLIN, R J.
Sustainable management of Bass Strait scallops... 0.0.0... 6 0.0. c cece cece ree een 307
CURRIE, D.R. & PARRY, G.D.
The impact of scallop dredging on a soft sediment community using multivariate techniques......... 315
BLACK, K.P & PARRY, G.D.
Field measurements of natural sediment transport rates and the sediment disturbance
due to scallop dredging in Port Phillip Bay ........... 0. cc cece ccc e rece nn 327
COVER, P. & STIRLING, D.
Scallop dredges: an engineering арргоасһ.............................+зз.8. жазса 343
HEASMAN, M.P., O'CONNOR, W.A. & FRAZER, A.W.
Improved hatchery and nursery rearing techniques for Pecten fumatus Reeve ..................... 351
O'CONNOR, W.A., HEASMAN, M.P., FRAZER, A.W. & TAYLOR, J.J.
Hatchery rearing the doughboy scallop, Chlamys (Mimachlamys) asperrimus (Lamarck)............ 357
SHUMWAY, S.E. & CEMBELLA, A.D.
Toxic algal blooms: potential hazards to scallop culture and ћѕһћепеѕ............................ 361
HART, B.
Dilemma of the boutique Queensland scallop... 6.0... ccc cece cece cere eect nese eet 373
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